Maverick Iranian clergyman Hojatolesam Mohammed Montazeri today described the recent Soviet venture into Afghanistan as a "Russian occupation" which will boost Iranian unity against all forms of imperialism.

Montazeri, who came to Beirut illegally Tuesday across the Lebanese-Syrian border, made the remarks at a hastily arranged press conference at the Abdel Aziz mosque, next door to the offices of the Palestine Liberation Organization. He literally had been chased there by Lebanese and Syrian security forces.

Montazeri told reporters that Iranian volunteers who had pledged to fight against Israel alongside PLO guerrillas were already in Lebanon. "Thousands would be flocking to Lebanon in coming weeks," he added. About 250 Iranian volunteers are currently in training at a PLO camp near Damascus.

Lebanon, however, has closed its borders to the Iranians, and Syria has pledged not to help them enter clandestinely, raising doubts about Montazeri's claim that some were already in this country.

The clergyman himself, son of a senior member of Iran's ruling council, was nearly caught by the Lebanese and Syrian forces at noon today as he prepared to speak at Beirut's Wiener House Hotel. They surrounded the building and fired volleys of machine-gun fire into the air as Montazeri rushed away to the mosque in a car.

He told reporters who found him there that "Iranian and Afghan Moslems will rise up together and fight to drive out all forms of outside intervention."

Montazeri also sounded a defiant note to the Lebanese government, saying his followers have "special means" of entering the country without visas.

Montazeri also said Iran's Revolutionary Council has approved $3 million in reconstruction aid for embattled southern Lebanon.