Thousands of Russians have gathered at the Sakharov Center in Moscow to attend the memorial service of former Russian Deputy Prime Minister Boris Nemtsov, who was gunned down by an unknown assailant near the Kremlin wall on Friday night.

Mourners are filing past the open casket in order to pay respects to Nemtsov’s mother, Dina Eydman, and his children Anton and Zhanna. The service was also attended by foreign dignitaries, including U.S. ambassador to Russia John Tefft and former UK Prime Minister John Major.

Nemtsov’s funeral will be held in the afternoon at Troyekurovskoye, a central Moscow cemetery.

55-year-old Nemtsov was a fierce critic of President Vladimir Putin’s policies and many believe that he was murdered out of revenge for his outspoken views. Several conspiracy theories have been circulating with regard to this mysterious crime, nurtured by the fact that Nemtsov was supposed to lead a rally in Moscow condemning Russia’s policies on Ukraine on the day of his death.

It should also be noted that, according to international media reports, several foreign officials have been barred from attending the funeral of the murdered Russian politician.

More specifically, The Guardian reports that officials from Poland and Latvia were denied entry into the country while the Polish foreign ministry said its Senate speaker, was barred from attending the funeral in reprisal for EU sanctions.