Lev Ponomaryov, head of the For Human Rights movement, told RFE/RL by mobile phone from a police station that he and four others had been detained after being "forcefully seized."
"When I got across from Tverskoi Boulevard to Sretensky [Boulevard] at noon, I was accompanied by my wife and three young people," Ponomaryov said. "Suddenly, our path was blocked by OMON [special forces police]. When we tried to pass by, without explanation they caught us and dragged us into a bus."
The police moved in as a group of activists were about to start a tour of the sites of the April 14 police crackdown on a big antigovernment demonstration.
Ponomaryov and the other four activists were released after spending three hours in custody.
He later told RFE/RL that he had been accused of heading an illegal procession and shouting, "Shame to OMON."
Lyudmila Alekseyeva, chairwoman of the Moscow branch of the Helsinki Group human rights organization, criticized the detentions as the "crudest violation of human rights."
She said the activists were not carrying posters nor chanting any slogans.
(with material from agency reports)