One of the protest leaders, Ararat Mirzoyan, wrote on his Facebook page that he had been illegally detained along with Mr. Pashinyan and a third opposition politician, as well as other protesters.

A witness told Reuters that police officers armed with batons and shields had cleared at least one area of the capital where protests were taking place, but that demonstrators remained in other areas.

Demonstrators continued to march in large numbers in the capital, blowing vuvuzelas and sounding car horns, despite earlier attempts by the police to break up the march.

The police called on protesters to stop the situation from escalating, the news agency Interfax reported, quoting an official statement. “If these demands are not fulfilled, the police has the right to take demonstrators into custody and to use force,” the statement said, according to Interfax.

Mr. Sarksyan said his opponents “did not learn the lesson of March 1,” referring to a protest rally after his re-election in 2008, in which 10 people were killed in clashes with the police.