Defying a harsh crackdown on political protesters in Iran, one of the country’s most outspoken opposition leaders on Monday called for a national referendum to gauge the popularity of the government.

The proposal from the leader, Mehdi Karroubi, was publicized on Iranian opposition Web sites after he had met with another opposition figure, Mir Hussein Moussavi, at Mr. Karroubi’s home in Tehran. Both men ran for president in the election last June and contend that the victory of the incumbent, Mahmoud Ahmadinejad, was a result of pervasive fraud.

It was clear from Mr. Karroubi’s call for a referendum that he did not expect the government to take it seriously. But his ability even to make such a demand and spread the message over the Internet seemed calculated to frustrate the Iranian authorities.

In his statement, Mr. Karroubi wrote that the authorities turned the country into a “military base” on Feb. 11 to propagate what he called the false image that the government enjoyed widespread support. He contended that the progovernment anniversary rally in Tehran’s central square that day was engineered by government propagandists.

“Despite efforts to engineer the rally by bringing in people with hundreds of buses and trains from around the country, the square remained half empty,” he wrote. “Do you think people do not understand the reason for the presence of tens of thousands of military forces on the streets?”

Fatemeh Haghighatjoo, a former member of Parliament who is a visiting scholar at the Center for Women in Politics and Public Policy at the University of Massachusetts, Boston, called Mr. Karroubi’s statement a form of psychological pressure on Iran’s top leaders.

“They have been claiming that the rally on Feb. 11 was a referendum and a sign of the government’s popularity,” she said, referring to the progovernment rally. “Now Mr. Karroubi is saying that if that is really true, why don’t you hold a real referendum.”