Huge crowds of people, some holding flowers, turned out in Moscow on Friday for the funeral services for Aleksei A. Navalny, Russia’s most prominent opposition figure, two weeks after his mysterious death in a remote Arctic penal colony.

The service was taking place under tight monitoring from the Russian authorities, who have arrested hundreds of mourners at memorial sites since Mr. Navalny died. Police presence was heavy around the church where funeral services began shortly after 2 p.m. local time.

People chanted Mr. Navalny’s last name as his coffin was taken into the Church of the Icon of the Mother of God Soothe My Sorrows, a Russian Orthodox church in southern Moscow. Images on social media showed attendees lining up, but also security cameras that the local news media reported had been recently installed, and signs forbidding mourners to take pictures or video in the church.

Ivan Zhdanov, who, like many of Mr. Navalny’s closest associates, is in exile outside Russia, encouraged people to come to the church, saying that the police had not been arresting mourners, as many had feared.

“People are coming to say farewell, and no one is touching them,” Mr. Zhdanov said. “Those who want to come to say farewell can do so.” Mr. Navalny’s supporters also created a website for supporters to light a virtual candle in his memory.

Almost 250,000 people were watching a livestream of the event organized by Mr. Navalny’s allies, while about 150,000 watched coverage on YouTube by the independent TV Rain, according to figures provided by the streaming platform.