An Iranian religious leader, said to be a leading member of the powerful Revolutionary Council under Ayatollah Ruhollah Khomeini, was assassinated last night in central Tehran.

A group called Forqan said it was responsible for killing Ayatollah Morteza Motahari. In a phone call to a local newspaper, the group also claimed responsibility for the assassination last week of the provisional government's former military chief of staff, Gen. Mohammed Vali Gharani.

Ayatollah Motahari, said to be in his 70s, was shot in the head as he left a dinner party in downtown Tehran late last night. The government radio reported the assassination early today and cited Forqan's phone call to the paper claiming responsibility.

The assassination of Gharani and Motahari raised fears that the group has embarked on a wave of terror against Khomeini's new Islamic government to promote its still undefined aims.

The ideology of Forqan-a Koranic term which means "Holy Book" or "the difference between right and wrong" in Arabic-is unclear, but it cited opposition to a "mullah's dictatorship" as the main reason for killing Gen. Gharani. A mullah is a Moslem religion leader.

After the general's assassination, a representative of the group left a leaflet at the offices of the Tehran newspaper Ayandegan listing six reasons for his killing. It accused him of cooperating with "imperialism" with alleged eforts for a pro-American coup, and with what it called "antimonotheism."

It also cited Gharani's efforts to create "an antinational army" and actions to crush Kurdish separatists. Using a term repeatedly employed during the trials of former officials under deposed shah Mohammad Reza Pahlavi, Forqan also accused Gharani of "corruption on earth."

Authorities arrested several suspects after Gharani's murder, but the killers are believed to be at large.

The latest victim, Ayatollah Motahari, is believed to be an associate of Ayatollah Mahmoud Taleghani, the most liberal of Iran's top Shiite Moslem leaders. Motahari's relationship with the shadowy Revolutionary Council could not immediately be confirmed, since the council's membership is a closely held secret.

Iran Radio said Motahari was the head of Alah Hyat University, a religious training institute in Tehran.

The Shite leader was shot in Jaleh Square where last September the shah's Imperial Guard shot at a crowd of demonstrators, killing hundreds. That shooting is regarded as one of the key acts that led in February to the shah's ouster.

Earlier yesterday, scuffling broke out between rival leftist and Islamic groups that had been holding marches in Tehran in observance of May Day.

The government radio reported that three former police officers had been executed after an Islamic court in the northern city of Gorgan convicted them of being "corrupt on earth." The unofficial total of persons executed for actions carried out during the shah's rule is now 164.