A demonstrator holds images of victims of the Polish jet crash, outside Russia's embassy in Warsaw on April 9 2010
Poland's opposition leader laid flowers Wednesday in honour of his twin and ex-president Lech Kaczynski, who died in a jet crash in Russia three years ago that one third of Poles believe was an assassination.
Jaroslaw Kaczynski and members of his conservative Law and Justice (PiS) party sang the national anthem with several hundred people at the presidential palace to honour the 96 victims of the crash.
"Wake up Poland! The Katyn and Smolensk crimes: attacks on an independent Poland," said signs at the rally.
PiS believes the April 10, 2010 disaster was nothing short of an assassination, despite both Polish and Russian investigators concluding it was an accident.
Scores of senior Polish statesmen died when the Russian-made Tu-154 airliner went down in thick fog while approaching Smolensk airport in western Russia.
The delegation was en route to memorial ceremonies in Katyn for thousands of Polish army officers slain by the Soviet secret police in 1940, a massacre the Kremlin denied until 1990.
PiS has repeatedly accused Tusk's centre-right government of not taking Russia to task over what it insists was a botched investigation and cover-up.
In a new report published Wednesday, a parliamentary group led by PiS legislator Antoni Macierewicz highlighted its "conviction that the catastrophe followed explosions".
The analysis ignores Polish prosecutors' conclusions that no explosive traces were found on the wreck of the Russian-made presidential jet.
"It turns out that... Russian intelligence services decided which firms and when Poland's most important planes would be overhauled," Macierewicz said in the report, quoted by the Polish press agency PAP.
According to a survey published last month, 33 percent of Poles said they "would not exclude" the possibility of an assassination.
Poland's Prime Minister Donald Tusk knelt before the graves of around 30 of the crash victims at the capital's Powazki Cemetery on Wednesday morning.
Lech Kaczynski's daughter laid flowers at Wawel Castle in Krakow, where her father and mother Maria, who also died in the crash, are buried.
"My pain is as fresh as it was three years ago," Marta Kaczynska told reporters.
Around 300 supporters of Polish nationalist groups rallied late Tuesday at the Russian embassy in Warsaw, accusing Russian and Polish leaders of having a hand in the accident.
They also called on Moscow to stop foot-dragging and hand over the plane's wreckage to Poland.
"It was not an accident. I understood that from the moment I turned on the TV. They're telling us nonsense," Warsaw resident Wieslaw Zukowski told AFP.
"In Russia, nothing happens by accident," he added.
Official ceremonies, along with demonstrations by the opposition, are planned throughout the day.
In an August report, Polish investigators said most of the blame for the crash lay with Poland but also faulted Russia for the sub-standard Smolensk airfield and poor traffic control.
But it ruled out "extremist versions" of events, including sabotage and third-party pressure on the crew to land despite bad weather.
However, a Russian probe from 2011 concluded that the crew was under "psychological pressure" to land in very poor visibility.