Poland's prime minister and new president laid a wreath at a Warsaw cathedral on Sunday, April 10, marking the one-year anniversary of a plane crash in Russia that killed President Lech Kaczynski and 95 other top Polish officials.

The ceremony marked the start of a day of national mourning, but was further clouded by tensions with Moscow over a plaque erected at the crash site in Smolensk in western Russia.

The original version of the commemorative sign noted in Polish that Kaczynski and the other crash victims had been travelling to the area to mark the 70th anniversary of the murder of 22,000 Polish officers by Soviet secret police.

But that plaque has since been swapped for a shorter version written in Russian and Polish, which makes no mention of the reason for the ill-fated visit. The massacre has been a source of intense friction between Poland and Russia, with the Kremlin only recently acknowledging the murders as a Stalinist crime.

Diplomatic row

The Polish Foreign Ministry has demanded an explanation for the changes and summoned Moscow's ambassador to Warsaw to express its dismay.

"This was a very bad decision which has spoiled not only the current commemorations but also bilateral relations," said Foreign Ministry spokesman Marcin Bosacki.

President Branislaw Komorowski was also expected to raise matter with his Russian counterpart Dmitry Medvedev on a visit to Smolensk and Katyn on Monday.

A Russian Foreign Ministry official dismissed the Polish complaints, saying Warsaw had been informed in advance that the plaque would be replaced.

"In this connection, the comments of the official representative of the Polish Foreign Ministry cause bewilderment," an unnamed official said.

A brother's anger

The day of mourning was also marred by deep political and social divisions within Poland.

Opposition leader Jaroslaw Kaczynski, twin brother of the deceased president, refused to attend the state ceremonies, instead placing a wreath in front of the presidential palace. Kaczynski has been critical of the government's handling of the crash and aftermath.

Protests were also held throughout the day by demonstrators who believe the government has been too soft on Russia in the way it dealt with the accident. A large crowd waving Polish flags gathered in front of the presidential palace to voice their disapproval.

During investigations into the crash, both countries found cause to blame the other side, and many Poles feel a proper explanation of how the presidential plane crashed was still missing.

Around Poland, thousands filled churches on Sunday to mark the crash anniversary, while the victims' families gathered for a Mass at Warsaw airport, where 96 coffins had been returned to Poland last year.

Permanent scar

A ceremony was also held on Saturday at the site of the crash in Smolensk, attended by the first lady of Poland, Anna Komorowska. She was joined by more than 100 relatives of the victims.

"We are in a place that will leave a permanent scar on our memory," Komorowska said.

"I'm shaken, I'm still shaken," said Pawel Deresz, whose wife, a member of Polish parliament, was killed in the crash. "I'm here for the second time since the crash and I still cannot believe it."

Author: Darren Mara, Joanna Impey (AFP, AP, dpa, Reuters)

Editor: Toma Tasovac