Sky News witnessed a pregnant woman being stretchered from the scene and footage later emerged of distressed and injured children who had been in the line of fire when police used tear gas and water cannon.

A nearby medical centre in Serbia reported that two people had been seriously injured and up to 300 sought treatment.

Some 1,500 refugees who had been hoping to travel through Hungary are now blocked by a 3.5m-high fence.

Tensions spilled over at about 3pm when some migrants responded to the police tactics by throwing rocks and house bricks.

The majority ran desperately from the scene.

Sky News' Colin Brazier described the scene at the Roszke-Horgos border crossing as clashes broke out around him.

"It's very fractious," he said. "There are young men who are really angry. It's a determined hard core of maybe a score - and behind them children and mothers.

"The imagery of this is being beamed around the world - people are going to look at this in other European capitals and be concerned."

Brazier said refugees had been left at the fence without food, water and information.

Hungary arrested 29 migrants during the riot, including one it described as a "terrorist".

Serbia sent ambulances to the crossing, where thick smoke billowed after the clashes.

The country reacted furiously to Hungary's tactics.

"This is being thrown across the border line, which no state has the right to do and because of that I protest in the strongest terms," Serbian minister Aleksandar Vulin said.

Serbia later said it would send extra police to the crossing to "prevent further attacks on the Hungarian police from our territory and in a humane and respectful way distance the migrants from the fence and the Hungarian police".

It is such a new route that the refugees have no idea what to expect.

All they do know is that Hungary is inhospitable and now violent and dangerous.

It has never been very welcoming but now it is shut and intends to defend its borders with as much force as its government deems necessary.

Fences, water cannon, gas, the military and the law are all being deployed with equal vigour.

On buses and in taxis they arrive at an informal junction on the old road to Croatia outside the town of Sid.

They shoulder their bags, grab a packed lunch from waiting NGOs and head off on a farm track through cornfields to yet another unofficial crossing point.

Some have done this through four or five nations.

By coming through Croatia and Slovenia to Austria, what should have been the last and arguably quickest leg of their journey has now been doubled.

I spoke to a number of the refugees who had made it to the Hungarian border.

They are all uniformly unhappy.

"It is terrible the way we have been treated," a young man from Damascus told me as he trudged with a group of friends towards the border.

"It is wrong, there are women and children in this heat, without food, without any respect being shown to them.

"Hungary is a big problem but we will keep going this way (through Croatia) and then eventually we will get there."

The Serbian authorities have shown remarkable flexibility in dealing with the tide of refugees but they still insist on keeping the official border crossings open, so they direct the refugees to what are basically illegal crossing points.

I've been on the road, following the migration route through Hungary, Austria and into Germany.

I wanted to find out more about the people travelling through Europe. Who are they? What is their profession? How did they get here? Where are they going?

And to ask: What drives a person to leave their home and everything they know behind and set off on a very long journey in search of a new life?

Here are some of the stories I found along the way.

RAAFAT: 37

At the border crossing from Serbia into Hungary I meet Raafat from Damascus in Syria. He wants to build a new future with his son in Norway. His journey has taken him via Lebanon and Turkey, where he says he swam at night for seven hours to reach Greece and Europe.

FADI: 38

I find Fadi taking shelter from the rain in a tent near Roszke. He's a professional artist from Damascus in Syria. He's travelling with his Nephew and wants to go to Germany. He left Syria 9 months ago, and took a boat from Turkey to Greece before travelling up through Macedonia and Serbia.

MUSTAFA: 14

In the refugee camp at Roszke in Hungary, 14-year-old Mustafa tells me his story through the fence. He's from Idlib in Syria where he says he went to school every day, until the building was destroyed by a bomb. He wants to go to Germany to be "free" and go to school.

AHMED: 19

At the border crossing between Hungary and Austria 19-year-old Ahmed is travelling with his mother, brother and little sister. He says they lost their home in Aleppo in Syria and escaped to Europe via Turkey by boat. He wants to go to Germany to study and become a dentist or a doctor.

The 11 candidates, selected on the basis of recent polls, went head-to-head at the Ronald Reagan Presidential Library in Simi Valley, California.

Senator Rand Paul said Mr Trump's habit of hurling personal insults made him "worried to have someone like that in charge of the nuclear arsenal".

Mr Trump hit back, saying: "I never attacked him on his look, and believe me, there's plenty of subject matter right there."

The mudslinging didn't end there. Mr Trump threw a jibe at Jeb Bush, saying he liked that he was "showing energy", having previously said that he lacked it.

Former Hewlett Packard CEO Carly Fiorina gave a strong performance with detailed answers on foreign policy and powerful attacks on Hillary Clinton's achievements.

She attacked Mr Trump's business acumen, reminding the millions of viewers that he filed for bankruptcy four times.

She said: "That is precisely the way you ran your companies.

"You ran up mountains of debt, as well as losses, using other people's money. Why should we trust you to manage the finances of this nation?"

Earlier they bickered over Mr Trump's recent insult to Ms Fiorina, saying people might not vote for her because of her face.

"I think women all over this country heard very clearly what Mr Trump said," she said.

Mr Trump's reply got a more muted response: "I think she's got a beautiful face, and I think she's a beautiful woman."

Mr Trump also sparred with former Florida Governor Jeb Bush, with Mr Bush hitting back at criticism of his brother George's presidency by saying: "One thing about my brother - he made us safe."

The epic debate covered a swathe of issues from the Iran nuclear deal to defunding planned parenthood.

Real estate tycoon Mr Trump, retired neurosurgeon Ben Carson and Ms Fiorina have enjoyed a recent rise in the polls thanks to a growing contempt for the perceived establishment elite.

Ahmed Mohamed, 14, was led away in handcuffs from Irving MacArthur High School in North Texas on Monday after bringing the device to his engineering class.

In a tweet posted on Wednesday, Mr Obama said: "Cool clock, Ahmed. Want to bring it to the White House?

"We should inspire more kids like you to like science. It's what makes America great."

White House press secretary Josh Earnest said Ahmed had been "failed" by his school, calling the episode a "teachable moment".

Facebook founder Mark Zuckerberg also invited Ahmed, an aspiring inventor who builds his own radios, to the social network's California headquarters.

"Having the skill and ambition to build something cool should lead to applause, not arrest," said Mr Zuckerberg.

"The future belongs to people like Ahmed.

"Ahmed, if you ever want to come by Facebook, I'd love to meet you. Keep building."

Irving Police Chief Larry Boyd told a news conference on Wednesday that Ahmed would not be charged.

"We have no evidence that there was an intention to create alarm," he told reporters.

The police department earlier said three teachers at the high school had accused the teenager of a bomb hoax.

Irving Independent School District spokesperson Lesley Weaver said students and staff are encouraged to report any suspicious behaviour.

"We will take all necessary precautions to protect our students and keep our school community as safe as possible," Ms Weaver said in a statement.

Ahmed said he demonstrated his homework project to his engineering teacher and was advised not to show it to anyone else.

When his clock rang in English class later in the day, he said the teacher confiscated it claiming it looked like a bomb.

Ahmed said he was led into a room where five police officers were waiting, one of whom remarked: "Yup. That's who I thought it was."

He was taken to Irving police headquarters to be questioned, fingerprinted and photographed, reports the Dallas Morning News.

"It made me feel like I wasn't human," Ahmed told the newspaper. "It made me feel like a criminal."

The principal reportedly threatened to expel him unless he made a written statement to police. Ahmed has been suspended.

The incident has sparked an outcry on social media, where #IStandWithAhmed was trending on Twitter.

His older sisters have set up a Twitter account for him, @IStandWithAhmed, which has more than 40,000 followers.

Alia Salem, the director of the North Texas chapter of the Council on American-Islamic Relations, said the arrest "raises a red flag".

Democratic presidential candidate Hillary Clinton has joined the chorus of tweets supporting Ahmed.

"Ahmed, stay curious and keep building," she wrote.

Irving Mayor Beth Van Duyne defended school officials, insisting they were simply following protocol. 

She made headlines in March when she accused Muslims of plotting to bypass US courts by offering shariah-law mediation to worshippers.