LONDON, JULY 18 -- A car bomb exploded today and injured a leading opponent of the Iranian government as he drove through a busy shopping area.

A telephone caller to the Press Association, Britain's domestic news agency, said a group called Guardians of the Islamic Revolution said it had planted the bomb.

"We are responsible and we will kill all the . . . monarchists," the caller said.

Amirhussein Amir-Parviz, a cabinet minister in Shah Mohammad Reza Pahlavi's government, escaped through the window of his heavily damaged sedan after the bomb exploded under the front seat.

He was taken to St. Stephen's Hospital in nearby Fulham. Scotland Yard officials said he suffered a broken leg, cuts and bruises. The bombing occurred less than a mile from Kensington Palace, home of several members of the royal family.

Friends said Amir-Parvis was chairman of the National Movement for Iranian Resistance in London, which opposes the government of Ayatollah Ruhollah Khomeini.

He held several ministerial posts in the shah's government and was agriculture minister when the shah was deposed in January 1979, said a friend, journalist Hossein Ghauimi.

A cousin of the victim, identified only as Mr. Amir-Parviz, was quoted by the Press Association as saying, "It is a nonviolent organization, but my cousin has received several telephone threats over the last few years."

"It is a fair assumption that this attack was political, although the man is not a diplomat," said Cmdr. George Churchill-Coleman, head of Scotland Yard's Antiterrorist Squad.

"The explosion went off under the front passenger seat," he said. "We assume it was . . . some sort of improvised explosive device strapped underneath the car."

The explosion was the latest in a series of Iranian-linked bomb blasts that have hit London since 1981, aimed at both pro- and anti-Khomeini factions.