Smolensk: Russian investigators focused on the pilots’ last decisions as a plane crash which killed Poland’s president and wiped out its military leadership plunged the Polish nation into deep mourning on Sunday.

Tens of thousands of people poured onto the streets of Warsaw and other Polish cities to show their grief after the jet came down in thick fog near the Russian city of Smolensk, killing 97 people including President Lech Kaczynski and a host of top state, military and religious officials.

Poland started a week of national mourning and the president’s identical twin brother, former prime minister Jaroslaw Kaczynski, was among the first relatives to go to Russia to identify the victims.

A nationwide minute of silence was to be held at midday. Russia ordered a day of national mourning on Monday.

Russian Prime Minister Vladimir Putin is leading an official investigation into the crash on Saturday, but Russian officials have already said the Polish pilots ignored air traffic control warnings that they were too low.

The Russian-built Tupolev Tu-154 jet was taking the presidential party to a memorial service for 22,000 Poles massacred by Soviet troops in World War II when it hit tree tops in fog while approaching Smolensk airport, officials said.

The symbolism of the president’s mission to a country which has long been a rival of Poland only added to the national shock over the deaths.

Tens of thousands of mourners gathered in front of the presidential palace in Warsaw and the nearby Pilsudski Square. They prayed, sang the national anthem, waved the red and white Polish flag and lay a carpet of candles and flowers in the national colours.

Kaczynski’s populist nationalism often made him a divisive figure at home and in Europe. But the nation united in grief.

“It’s my heart that told me to come here, I simply had to come," said 45-year-old translator Anna Ciostek, who went to the square with her daughter and mother.

Bronislaw Komorowski, the head of Poland’s lower house of parliament who took over as interim head of state, joined the throngs in Pilsudski Square.

“The unprecedented scale of this tragedy is unique on a global scale," he told AFP. “We have to deal with this very difficult problem, and we will deal with it."

Polish Prime Minister Donald Tusk and the late president’s twin brother went to the crash site late on Saturday where they were met by Russian Prime Minister Vladimir Putin.

He returned to Warsaw on Sunday and cancelled a visit to Washington for a nuclear safety summit this week, as well as a following visit to Canada.

The body of the president will be transported directly to Warsaw from the scene of the crash, RIA Novosti news agency said, quoting an unnamed local government source.

The remains of the other victims will be sent to Moscow for identification, it added.

Besides the 60-year-old head of state, the crash killed his wife Maria, Poland’s military chief of staff and the heads of the main services, its central bank governor, the deputy foreign and defence ministers, an archbishop and members of parliament.

“Everything must be done to establish the reasons for this tragedy in the shortest possible time," said Putin.

The Tu-154 had already made three approaches to Smolensk airport when it crashed in the fog, according to witnesses and airport officials. At least one other plane had been diverted to another airport.

The jet “clipped the tops of the trees, crashed down and broke into pieces," regional governor Sergei Antufiev said.

Lieutenant General Alexander Alyoshin, deputy head of the Russian air force, said the Polish pilots had ignored warnings from air traffic controllers that they were too low.

“The head of the (air traffic control) group ordered the crew to return to horizontal flight, and when the crew did not fulfil the instruction, ordered them several times to land at another airport," he said.

“Nonetheless the crew continued to descend. Unfortunately this ended tragically."

Wreckage was scattered across a forest, but the black-box flight recorders were quickly found.

The crash was only a few kilometers (miles) from the Katyn Forest where Kaczynski and his delegation were to have attended a memorial service for the Polish officers and troops killed in 1940 by Soviet troops acting on orders from Josef Stalin.

The service was intended to help reconciliation between Poland and Russia, two decades after the end of the Cold War and the ensuing demise of communism.

Polish communities around the world held their own special church services for the president and the European Union also declared Monday a day of mourning. Global leaders around the world expressed condolences to Poland.

US President Barack Obama remembered “a distinguished statesman... widely admired in the United States as a leader dedicated to advancing freedom and human dignity."

German Chancellor Angela Merkel was “deeply shocked", while French President Nicolas Sarkozy hailed Kaczynski as a man “driven by ardent patriotism, who dedicated his life to his country."