Two trains travelling on the same track collided on Saturday evening near the north of Krakow in southern Poland, killing at least 16 people and injuring 60 others.
The head-on collision happened during maintenance work on the Warsaw-Krakow route, as a result of which one train ended up on the wrong track.
One of the trains was travelling from Przemysl, near the Ukrainian border to Warsaw, while the other was southbound from Warsaw to Krakow.
The trains were carrying about 350 passengers when the accident happened.
Officials said it was too early to confirm the exact cause of the crash but human error could not be ruled out.
More than 450 firefighters and other rescue workers had been rushed to the accident site to rescue victims from the wreckage.
According to reports three coaches are especially damaged and the rescue operation is focused on finding passengers in the mangled coaches.
Poland Prime Minister Donald Tusk said that several of the passengers were foreigners, including people from Ukraine, Spain and France, but none of them were among the dead or mostly seriously injured.
Following the accident Poland announced a two-day national mourning period on Sunday.
The disaster took place just three months before the Euro 2012 championships where millions of football fans have started travelling to the country, many of them by train to watch matches which are being co-hosted by Ukraine.