MOSCOW, October 7 (RIA Novosti) - A rally was held in central Moscow on Wednesday in memory of Russian investigative journalist Anna Politkovskaya, who was shot dead by an unknown killer three years ago.

Politkovskaya's friends and colleagues observed a minute of silence, holding posters that read: "Three years have passed since the murder. The mastermind and the killer are still at large."

"Only if we always remember the journalists who have been killed and only if we preserve the memory of them will we be able to protect those who continue doing what Anna Politkovskaya and Natalya Estemirova were doing," said Lyudmila Alexeyeva, chair of the Moscow Helsinki Group.

Estemirova, a researcher for the Memorial human rights group, was shot dead in Russia's North Caucasus earlier this year.

Representatives from the Reporters Without Borders organization claimed on Monday that they were refused visas by the Russian authorities to attend the event.

In February, a Moscow court acquitted three men charged with involvement in the shooting of Politkovskaya in her Moscow apartment building, but the Supreme Court overturned the ruling and ordered a retrial.

Novaya Gazeta reporter Politkovskaya, who gained international recognition for her reporting of atrocities against civilians in the troubled North Caucasus republic of Chechnya, was gunned down in an elevator in her Moscow apartment building on October 7, 2006.

Then President Vladimir Putin said that the killing, which took place on his birthday, had been carried out by forces seeking to "create a wave of anti-Russian sentiment in the world."