About 150 Shiite Moslem riflemen stormed Saudi Arabia's Embassy today, smashing windows, destroying documents and burning portraits of Saudi monarchs.

Soldiers of the Lebanese Army's mainly Shiite 6th Brigade watched passively as the pillaging went on. The embassy compound, in predominantly Moslem west Beirut, has been closed since February. Saudi diplomats were evacuated when Shiite militiamen fighting in the streets defeated the Army, which had split along religious lines.

Earlier today, two rocket-propelled grenades were fired into the British Consulate in west Beirut, causing extensive damage. Reuter said a group called the Lebanese National Resistance Front claimed responsibility, charging that Britain was facilitating the emigration of Lebanese from Israeli-occupied southern Lebanon. Britain noted that it briefly had represented Brunei in such immigration matters.

The gunmen who sacked the Saudi Embassy were part of a demonstration protesting alleged Saudi procrastination in issuing visas to Shiites wishing to go to Mecca on the annual religious pilgrimage. Lebanese must apply for visas in Damascus, Syria, with the Saudi offices here shut.

The visa applications sometimes take months to process. Shiites charge that they are screened more carefully than others to prevent pro-Iranian activists from entering Mecca, Islam's holiest city.

Saudi Arabia is ruled by the orthodox Moslem Wahabi sect, keeper of Islam's holiest places. Its two-story embassy building was set afire and gutted in this latest manifestation of rising Shiite fervor here. The attackers pasted up posters of Iran's Ayatollah Ruhollah Khomeini and shouted, "Khomeini, your wish is our command."

Viewers said the Saudi flag was ripped down and set ablaze, and an Iranian flag hoisted in its place. Chanting "God is great!" and "Down with the house of Saud!" the gunmen screeched up aboard cars and motorcycles. Soldiers later cordoned off the area.

Last week, Iranian Charge d'Affaires Mahmoud Nourani returned to Beirut to a tumultuous welcome by 3,000 pro-Iranian Shiites, who look to Tehran as Catholics look to Rome. Lebanon had cut off diplomatic relations with Iran last year because of the illegal presence of Iranian Revolutionary Guards in its central area controlled by Syria, but the ties were restored under Syrian and Shiite pressure.