At least 44 people were killed when a passenger train plunged into a ravine in southern Montenegro.
Many of the casualties are thought to be children
More than 180 people were injured one of the worst rail accidents in Montenegro's history.
The disaster happened shortly after 1600 (1500 GMT) on Monday at Bioce, about 10km (six miles) north of the capital Podgorica.
Interior Minister Jusuf Kalamperovic said it was caused by a failure in the train's braking system.
The train - carrying about 300 people from the northern town of Bijelo Polje to the port town of Bar - plunged into the 100m-deep (330ft-deep) ravine full of rocks.
Ninety of the injured are said to be children. Many were returning from a skiing trip.
The government has declared three days of mourning and Transport Minister Andrija Lompar has offered his resignation over the accident.
Helicopters hovered over the site as rescuers struggled amid bad weather to reach the stranded survivors.
"I had fallen asleep when a loud noise woke me," survivor Stanislava Bukovic told the Associated Press news agency.
"Then I felt something hit my head and lost consciousness. The next thing I knew I was on this stretcher," she said.
Karman Chofu, another passenger, said: "It was horrible, I saw many dead and wounded around me."
Montenegrin President Filip Vujanovic and Prime Minister Milo Djukanovic rushed to the crash site.
"A terrible tragedy happened at Bioce and everything is being done to reduce the number of casualties as much as possible," President Vujanovic told reporters.
"The whole republic has been mobilised to save lives. We are all hoping that this terrible accident will have as few casualties as possible," he added.
Serbia and Montenegro's rail network has suffered from years of under-investment but is used by much of the population simply because it is the cheapest method of travel, the BBC's Balkans correspondent Matt Prodger says.
Montenegro is the smaller of two republics making up Serbia and Montenegro, the only two republics of the former Yugoslavia still joined together.
It has a population of about 620,000.