A demonstration against the Iranian government planned more than a month ago by an opposition group, People's Mujaheddin, took a different turn yesterday in light of the devastating earthquake that hit northern Iran early Thursday.

The 1,200 protesters in Lafayette Square mixed cries for the overthrow of the Iranian government with expressions of sympathy and concern for the victims of the earthquake. Some said they had been trying vainly to contact the quake area by phone and had not learned of the fate of relatives.

The rally started with loud denunciations of the Iranian government and calls for freedom and democracy in Iran, followed by a silent prayer for the earthquake victims. Critics of the People's Mujaheddin say the group is a leftist organization and the U.S. State Department has described it as terrorist. The group denies the terrorism charge and says it is pro-democracy.

Waving Iranian flags and chanting "Iran, death to Khomeini's heirs, Iran," demonstrators denounced what they called the "terrorism and internal repression" of the Iranian government.

Conditions in Iran "make Iranian people vulnerable to disasters like earthquakes," said Alireza Jafarzadeh, a spokesman for the People's Mujaheddin. "Natural resources have been put in favor of war. The quality of housing, density of the population, and area that people are living in were way below the limit and that is why you see such high casualties."

The estimated death toll in Iran's worst earthquake, which struck the Caspian Sea region, had risen to 40,000. People's Mujaheddin has estimated higher figures than the government since the quake.

People traveled here from across the nation for the protest and march, which began at Lafayette Square, headed north to Dupont Circle, then returned to Lafayette Square. Some marchers said they had taken off work and had spent hundreds of dollars and long hours to get to the rally.

An Iranian college professor from Seattle named Bijan, who asked that his last name not be mentioned for fear of endangering family members, said he hopes the "demonstration will show the people of America and the Iranian regime how strong resistance is and that these are the last moments for the regime."

Bijan's wife, Karen, said she hopes that, despite tensions between the United States and Iran resulting from the hostage crisis of 1979 to 1981, Americans "can look beyond that and help the earthquake victims like they did for Armenia. What this government has done is not reflective of the people in Iran."

Rep. Mervyn Dymally (D-Calif.) spoke to the protesters in Lafayette Square and said he is pleased that the United States has offered support to Iran but that he hopes aid will be sent to private organizations and not to the government.

While Dymally said the U.S. government should immediately focus on helping earthquake victims in northern Iran, he urged that attention next turn to doing "everything in our power to get rid of the . . . regime that now exists in Iran."