By Calin Neacsu with Eric Randolph in Tovarnik

Röszke (Hungary) (AFP) - Hungarian riot police fired tear gas and sprayed water cannon Wednesday at huge crowds of migrants desperate to cross the border from Serbia, as other groups of refugees carved out a new route through Croatia.

Tensions boiled over when hundreds of furious refugees and migrants tore down wire meshing across two blocked access routes to Hungarian territory at the flashpoint Roszke crossing, injuring 14 Hungarian police officers.

The Serbian government lodged a formal protest with Hungary over the use of tear gas on its territory, and Interior Minister Nebojsa Stefanovic vowed to send police reinforcements to the Serbian side of the border to help calm tensions.

UN chief Ban Ki-moon said he was "shocked" by Budapest's actions, as hundreds of people fleeing war and misery, many of them Syrians, remain stranded at Hungary's newly fenced-off border.

"We want to leave! We want to leave to Germany!" cried one French-speaking man at a migrants' protest at the border through a megaphone. "Open the door!" he added in English, with hundreds echoing his call.

Crowds who managed to overrun police lines and break through the fence at Roszke did not take advantage to run deeper into the central European country's territory however, apparently wanting instead to show their frustration after Budapest sealed the border on Tuesday, an AFP correspondent said.

The Hungarian government said that 14 police were injured in clashes against stone-throwing migrants. Police responded with water cannon and tear gas.

Gyorgy Bakondi, Hungarian Prime Minister Viktor Orban's chief advisor, said the tough response came an hour after the migrants had issued an "ultimatum" to police, demanding to be let through.

"We will repair the fence, in fact we will put up a stronger fence," he told a news conference.

In a further show of force, Hungary deployed three military vehicles mounted with guns some 100 to 200 metres (yards) from the border, an AFP reporter at the scene said.

The UN refugee agency has criticised Hungary's hardline anti-migrant stance, saying it could violate the 1951 Refugee Convention.

- 'Not acceptable' -

"I was shocked to see how these refugees and migrants were treated. It's not acceptable," UN Secretary General Ban Ki-moon told reporters when asked about the border clashes.

Hungary has since Tuesday all but sealed the razorwire-topped border while threatening three-year jail sentences against anyone who tries to cross illegally.

Earlier Wednesday, migrants desperate to find new ways to eastern Europe were granted access by Croatia.

By Wednesday afternoon, a total of 892 migrants had entered the Balkan nation, an interior ministry statement said, with the government adding it was expecting another 4,000 in coming days.

- Solutions needed -

Pressure is building for a special EU summit to come up with solutions to the continent's worst migration crisis since World War II, with the bloc bitterly split and free movement across borders -- a pillar of the European project -- in jeopardy.

Croatian Prime Minister Zoran Milanovic said all migrants could pass through the EU state, allowing them to push on towards Slovenia, Austria and Hungary's fenceless southwestern frontier.

"We are ready to accept and direct those people, their religion and colour of skin is completely irrelevant, to where they apparently wish to go -- Germany and Scandinavia," Milanovic told lawmakers.

But his Slovakian counterpart Robert Fico added to the war of words between EU capitals by warning Europe was at risk from what he called a "migrant onslaught".

Amadou, 35, from Mauritania in west Africa told AFP he was now following the new route through Croatia.

"We heard that Hungary was closed so the police told us we should come this way," he said as he walked from the Serbian border town of Sid towards the Croatian frontier -- which is still peppered with minefields from the Balkan wars of the 1990s.

Orban has already said he was planning to build a fence along the frontier with Romania as well, prompting outrage from its southern neighbour.

Hungary's apparent success in deflecting the flow of migrants through its territory sparked fears in Serbia that it would be swamped by an unmanageable number of migrants.

Amnesty International warned in a statement 1,000 people were "stuck in abysmal and rapidly deteriorating conditions along a Serbian motorway after Hungarian authorities closed the border crossing."

- Domino effect -

From the Alps to Istanbul, thousands of other migrants were caught in similar bottlenecks, with hundreds setting out to walk to Germany from the Austrian border city of Salzburg after trains north were suspended following Berlin reintroduction of border controls.

Germany, Austria and Slovakia have all reimposed identity checks on parts of their borders, and Poland and the Netherlands are considering whether to follow suit.

Hundreds of migrants meanwhile were stranded in the Turkish border city of Edirne after police stopped around 1,000 refugees from crossing into Bulgaria, whose authorities are discussing the possibility of "sending in the army", and Greece.

Also in Turkey, huge crowds were camped out at Istanbul's main bus station for a second night running, after being refused tickets to Edirne.

- Blow to Schengen -

Politically, the big concern is for the future of the 20-year-old Schengen agreement, which governs borderless travel between member states, and is considered as important as the euro by many EU supporters.

By doing away with border checks and reducing bureaucracy, it provides a powerful economic stimulus and enhances a common European identity, they say.

Berlin's decision Wednesday to extend greater passport controls to its border with France -- the Schengen zone's other principal architect -- seemed to deal it a huge blow.

German Chancellor Angela Merkel and her Austrian counterpart Werner Faymann on Tuesday called for a special EU summit next week to debate the crisis.

"Time is running out," Merkel warned, urging an end to the squabbling that has flared since eastern members flatly refused to accept EU-set quotas for taking in tens of thousands of refugees at a contentious meeting on Monday.

European President Donald Tusk will announce a decision about the possible summit on Thursday, with EU interior ministers to meet again Tuesday in a fresh bid to resolve the quota wrangle.