Eurostar blamed "intruders" for blocking trains from entering the 50-kilometer (31 mile) tunnel. Several trains, each carrying several hundred people, were disrupted Tuesday night and into Wednesday morning.

Passengers complained of being forced to sit on dark, stifling trains for up to four hours after power was cut. Food and water were distributed and rescue services were put on standby for vulnerable passengers, including the elderly, said a company official.

The blockade followed a similar chaotic scene hours earlier, hundreds of kilometers away at Budapest's main international railway station, when hundreds of refugees attempted to board trains to Germany. The station was closed to migrants, forcing them to sleep outside the main entrance overnight, guarded by police.

"There must be about 1,000 people on the platforms, which are full to overflowing. There is no communication, soldiers are present," said passenger Geraldine Guyon. Another traveler said she thought migrants had climbed onto the roof of the train.

Several passengers tweeted their frustration. "We've been abandoned on a platform somewhere in the cold with NO INFORMATION," wrote one of the passengers, Danny Bell, after passengers were allowed to disembark one of the trains at Calais station some hours later.

"And enlighten me - at 4am in the middle of nowhere what are those options? For hundreds and hundreds of people?" he added.

Three trains continued their journeys to London, with two others forced to return to Paris. A sixth service was unable to be used to return to Paris due to a technical fault, so a replacement train was organized, said a tweet from Eurostar.

Over the summer, the French port of Calais and the Channel Tunnel have seen several incidents where groups of migrants have attempted to travel illegally to the UK. In July, hundreds of them tried to storm the tunnel entrance. On a daily basis, small groups of migrants attempt to climb into the back of lorries heading to the UK by ferry or the Eurotunnel freight rail service. Others have broken through security fences and onto the rail tracks that lead to the tunnel.

Britain has announced an extra 7 million pounds (9.8 million euros) to help France secure the Eurotunnel site on its side of the English Channel, in addition to 4.7 million euros already spent on erecting barriers aimed at securing access to the terminal and the platforms.

mm/kms (AFP, Reuters)