Warsaw, Poland (CNN) -- The body of Polish President Lech Kaczynski, who was killed in a plane crash in Russia over the weekend, returned to tributes in his homeland Sunday afternoon.
Soldiers in perfect step carried the casket from the plane that transported onto the tarmac, where mourners were waiting. Catholic priests recited prayers at the military airport before Kaczynski's daughter and twin brother, followed by others, took turns kneeling before the flag-draped casket.
People lined up along the streets along where Kaczynski's body would pass on its way to the presidential palace.
Tens of thousands of Poles across the country observed a two-minute-long moment of silence to remember their president and 95 others killed in the plane crash.
Meanwhile, investigators said they found the aircraft's flight data recorders in good condition and began deciphering them Sunday, the independent Russian Interfax news agency reported.
Residents flocked to central Warsaw, the site of the presidential palace, to mourn Kaczynski. They left wreaths and lit candles. By Sunday, the numbers grew to about 100,000 by some estimates. Many cried openly. Others stared blankly at the sky.
On Monday, the country begins a week-long period of mourning.
The plane carrying Kaczynski crashed Saturday morning while trying to land at an airport near Smolensk in Russia.
Kaczynski's wife and several top military officials were also killed in the crash.
Parliament Speaker Bronislaw Komorowski took over as acting president and declared it "a time for national mourning."
Kaczynski, 60, had been traveling with a Polish delegation to Russia for the 70th anniversary of the Russian massacre of Polish prisoners of war in the village of Katyn.
About 20,000 Poles, including soldiers and civilians, were executed there during World War II.
The Polish military plane carrying Kaczynski originated in Warsaw, the Polish Defense Ministry said.
It was just a few miles east of Katyn when it crashed around 10:50 a.m. (2:50 a.m. ET) on the outskirts of the town of Pechorsk, close to Smolensk, the Investigation Committee of the Russian prosecutor's office said.
Late Saturday night, Kaczynski's twin brother, Jaroslaw, visited the site of the crash.
As others around him prayed, he silently knelt down on pile of rubble where mourners had propped up flowers.
Authorities do not yet know what caused the crash. Russia has emphasized that there is no evidence it was responsible.
The Investigation Committee said the plane, a Tupolev-154, was trying to land in heavy fog. A Russian military official said that air traffic control in Smolensk had tried to divert the plane to another airport because of inclement weather.
"The air traffic control officer gave several orders to divert but the plane continued with its descent," said Alexander Aleshin, a top Russian air force official. "Unfortunately, this ended in tragedy."
Russian President Dmitry Medvedev appointed Putin to head an inquiry commission.
On Sunday, the Russian state-run RIA Novosti news agency reported that the crash was not caused by technical problems, citing investigation officials.
The plane carrying Kaczynski was refurbished and repaired last year, according to Alexei Gusev, general director of Aviakor Factory, the company that performed the service.
"Speaking openly, we believe that this tragedy could not have been caused by equipment failure," he said.
Kaczynski had been president since December 2005, after he defeated rival Donald Tusk in the second round of voting.
Elections must now be held within 60 days.
Other Polish officials killed in the crash include Aleksander Szczyglo, the head of the National Security Office; Jerzy Szmajdzinski, the deputy parliament speaker; Andrzej Kremer, the deputy foreign minister; and Gen. Franciszek Gagor, the army chief of staff, according to Kaczynski's Law and Justice Party.
The party also said that Slawomir Skrzypek, head of the National Bank of Poland, was killed.
Putin spoke Saturday at the crash site, where charred pieces of the airplane were strewn through a wooded area. Some pieces, including one of the wheel wells, were upside down.
"As our first priority, we must establish the causes of this tragedy," he said. "As a second priority, we must do everything in our power to assist the families and relatives of the deceased."
CNN's Frederik Pleitgen in Warsaw, Poland, and Nic Robertson in Smolensk, Russia, contributed to this report.