Investigators are scrambling to determine what caused Saturday’s plane that killed Polish President Lech Kaczynski and 96 others, while Poland remains in shock a day after the tragedy.

The presidential party was en route to a memorial service for Poles massacred by Soviet troops in World War II when its Tupolev Tu-154 airliner crashed in thick fog while approaching Smolensk airport, in the west of Russia.

Besides the 60-year-old head of state, the mid-morning crash killed his wife Maria, as well as Poland’s military chief of staff, other senior officers, its central bank governor, deputy foreign minister and members of parliament.

Mourners poured out on to the streets in Warsaw on Saturday following the news, singing the national anthem, waving the white and red Polish flag, and laying a carpet of candles and flowers at the presidential palace and nearby Pilsudski Square.

Unprecedented tragedy

Polish Prime Minister Donald Tusk described the crash as "the most tragic event of the country's post-war history." He made the comments before heading to the crash site on Saturday, accompanied by the late president's twin brother and former prime minister Jaroslaw Kaczynski and Russian Prime Minister Vladimir Putin.

The remains of all the victims were recovered and transferred to Moscow in two heavy-lift military helicopters for identification.

Tusk has called a special cabinet meeting later this Sunday.

Europe in shock

Poland will hold two minutes of silence at noon on Sunday´and acting president Bronislaw Komorowski has declared a week of mourning in the country.

"We are united in the face of this huge tragedy. There is neither the right wing nor the left, there are neither divisions nor differences," he said in a televised address to the nation.

Komorowski said he would set the date of a presidential election which had been due in October after holding talks with Poland's political parties. Under the constitution the election must now be held by late June.

The European Union has declared Monday a day of mourning.

German Chancellor Angela Merkel said she had been shocked and lost for words when she first heard of the crash.

"This is a political and human tragedy for Poland our neighbour," Merkel told reporters at her Berlin office. Describing Kaczynski as a "combative European" who had loved his country, she said: "We in Germany will miss him too."

The news especially shocked Germany's large Polish immigrant community. Eight Polish handball players employed with German clubs wore black armbands during matches at a tournament in Hamburg's Color Line Arena.

Putin sends condolences

Offering their condolences to a nation in mourning, Russian Prime Minister Putin and President Dmitry Medvedev vowed a thorough investigation into the plane crash.

"This is first and foremost Poland's tragedy and that of the Polish people, but this is also our tragedy, and we mourn with you and grieve with you," Putin, who heads the inquiry commission, said in televised comments.

"I have given an order to carry out a detailed investigation of the accident with complete and close cooperation with the Polish side," said Medvedev in a message to Bronislaw Komorowski, speaker of the Polish parliament and acting head of state.

Earlier Saturday, Medvedev appointed Putin to head a special commission to investigate the crash.

"Everything must be done to establish the reasons for this tragedy in the shortest possible time," said Putin as he arrived in Smolensk to visit the crash site.

rb/cmk/dpa/AFP/Reuters

Editor: Sonia Phalnikar