Hundreds of Eurostar passengers have been left stranded in dark, after their train to London was stopped in its tracks by asylum seekers who are thought to have climbed onto the roof of the train.

The train from Paris slowed to a halt on Tuesday night as it was about to enter the Channel Tunnel and police arrived on the scene, including with a helicopter overhead, witnesses said, in a bid to stop asylum seekers who had got onto the tracks from going further.

A Eurostar official said water and food had been distributed to passengers on the 9055 train from Paris to London, which remained at the Calais-Frethun station in northern France at 5:00am (local time).

"Rescue services are also present for those who need help, particularly the elderly," added the official, describing the situation as "complicated".

Five other trains originally disrupted by the incident were either been sent back to where they departed from or on to their final destination.

The Calais-Frethun station lies close to the port of Calais, where some 3,000 asylum seekers living in makeshift camps have been ramping up their attempts to outfox hopelessly outnumbered security officials and police to reach what they see as the El Dorado of Britain.

Clothilde, a 23-year-old French woman who lives in London, said she had seen the police hurrying down the train and believed there were asylum seekers on the roof.

"We have not see the migrants, but we knew that they were everywhere on the roof and that's why we waited for a helicopter to ensure there were no migrants above us," she said, adding: "Passengers are not allowed to leave the station, except to take a taxi at the entrance."

The train was later towed back to the station nearest to the port of Calais, where thousands of asylum seekers living in slum-like conditions have made regular attempts to cross the tunnel to Britain.

Eurostar was unable to say exactly how many passengers were on board the train, but the trains carry a maximum of 750 people.

"We have significantly reduced the number of intrusions [into the Channel Tunnel]," French interior minister Bernard Cazeneuve said.

"Our aim is to reduce them to zero, and to let it be understood, particularly where traffickers are concerned ... that there cannot be any crossing," he told Europe 1 radio.

France and Britain have brought in emergency measures to deal with a surge in the number of people trying cross the Channel Tunnel after people were killed while making a desperate attempt to reach England.

Anger in Budapest as asylum seekers stopped from boarding trains to Germany

Asylum seekers wave their train tickets and lift up children outside the main Eastern Railway station in Budapest. ( Reuters: Laszlo Balogh )

Europe is in the midst of the biggest mass movement of people since the end of World War I, as thousands of people flee the fighting in Syria and other Middle East conflict zones.

Hundreds of asylum seekers protested in front of Budapest's Keleti Railway Terminus for a second straight day on Wednesday, shouting "Freedom, freedom!" and demanding to be let onto trains bound for Germany from a station that has been closed to them.

Chaos this week at the station in the Hungarian capital has become the latest symbol of Europe's migration crisis, the continent's worst since the Balkan wars of the 1990s.

More than 2,000 asylum seekers, including families with children, were waiting in the square at the station while Hungarians with IDs and foreigners with valid passports could board the trains.

Asked if Hungary would again let asylum seekers board trains to Germany as it did on Monday, a government spokesman said Budapest would observe European Union rules which bar travel by those without valid documents.

The station has been shut to asylum seekers since Tuesday morning.

"A train ticket does not overwrite EU rules," spokesman Zoltan Kovacs said.

"Please, we are human too," a sign in German held up by a young boy said at an earlier protest.

Refugee rights group Hungarian Helsinki Committee warned the situation at the station was "very tense and unpredictable".

Figures show hundreds of thousands heading to Europe

The latest flashpoint, one of several recent standoffs at borders and transport hubs across the continent, came as the International Organisation for Migration (IOM) published new figures revealing the scale of the crisis.

Out of the 350,000 arrivals by sea so far this year, 234,770 alone were in Greece, it said.

That figure by itself is more than the entire Europe-wide total for all of 2014.

At least 2,600 died trying to reach Europe, either by drowning or suffocating in packed or unseaworthy boats, the agency said.

Austrian police said on Tuesday they had freed 24 teenagers from Afghanistan from a van in Vienna which had been welded shut, and arrested the Romanian driver.

"It was like a rolling prison cell..they were crammed in, sitting and standing on top of each other," said Thomas Keiblinger, spokesman for the police in Vienna. "They would not have made it too much further."

AFP/Reuters