Athens/UNI: The death toll from the collision of two trains in central Greece on late Tuesday night has increased to 36, while 66 injured people were still hospitalized, including six in serious condition in intensive care units, the Greek Fire Service said on Wednesday.

More than 150 firefighters, including special units, were participating in an ongoing search and rescue operation for survivors in the wreckage, the spokesperson of the Fire Service, Vasilios Vathrakogiannis, told a press briefing.

Efforts are focused on the first three carriages of the passenger train that crashed onto a freight train close to midnight at Tempi municipality, he added. They were engulfed in flames after the collision.

A total of 346 passengers were on board the passenger train, national broadcaster ERT reported, citing the Hellenic Train rail operator.

An investigation into the cause of the accident has been launched, Greek police said.

Greek government spokesperson Giannis Oikonomou expressed the government's condolences for the tragedy and announced a three-day national mourning.

From Wednesday to Friday, flags will fly at half-mast on all public buildings and all public festivities will be suspended, according to an e-mailed statement.

Authorities will clarify how the two trains were moving on the same track for many kilometres from different directions, he added.

The passenger train was heading to the port city of Thessaloniki in northern Greece from the capital Athens, while the cargo train had departed from Thessaloniki for Athens.

The president of the train drivers' association, Kostas Genidounias, told ERT that there were shortcomings in the operation of the railways that should be addressed.

Electronic traffic control systems warning drivers of dangers ahead had not been working and the Athens-Thessaloniki network run in manual mode, he said.

As Greek Prime Minister Kyriakos Mitsotakis was travelling to the crash site, European Commission President Ursula von der Leyen expressed condolences for the tragedy.

"My thoughts are with the people of Greece after the terrible train accident that claimed so many lives last night near Larissa. The whole of Europe is mourning with you. I also wish for a speedy recovery for all the injured," she tweeted, adding "We stand by your side" in Greek.

The Hellenic Red Cross has called people to donate blood in a central square in the nearby city of Larissa to help the injured.