Thousands of angry Moslem fundamentalists marched on the main Christian cathedral of Cairo today after the most sweeping crackdown on religious extremists and political opposition elements since President Anwar Sadat came to power 11 years ago.

Eyewitnesses said security police stopped the mostly youthful demonstrators before they reached the grounds of the huge St. Mark's Coptic Cathedral in the downtown area by using five machine-gun-mounted armored cars to block off the main route of the march and then throwing tear gas into the crowd, estimated at its height at around 10,000.

The demonstration, the only major one reported in the country today, occurred following noon Moslem prayer services at the nearby half-built Mosque of the Light, a stronghold of the Moslem fundamentalist movement here. An official of the mosque, Hafez Salama, was among those arrested early yesterday in the government's nationwide sweep.

Meanwhile, the unofficial figure for the total number of arrests rose today to 680, including at least 16 Moslem fundamentalist and Christian Coptic leaders, a large number of political opposition figures of varying tendencies, lawyers, journalists and other outspoken opponents of the Sadat regime.

Later, the first Saturday edition of the semiofficial daily Al Ahram reported the number of detainees had risen to 1,100, by far the largest number of political arrests ever carried out under Sadat.

Perhaps the best known detainee abroad among Sadat's opponents is Mohammed Heikal, 58, who was a longtime confidant of the late Egyptian leader Gamal Abdel Nasser and a former editor of Al Ahram.

Virtually every independent publication critical of Sadat's foreign or domestic policies has been outlawed, the state-controlled media disclosed.

The local press said six publications had their licenses revoked because they had helped to foment the recent upsurge in Moslem-Christian sectarian strife, which has taken the lives of at least 21 persons in the last three months.

With the closure of these newspapers, Sadat has placed himself in a position to effectively prevent any public airing of views opposing his policies outside the parliament and the state-controlled media.

So far, however, the government has justified the crackdown on the ground of the need to bring an end to the sectarian strife afflicting this nation of 43 million people. Although most Egyptians are Moslems, there are also 3 million to 6 million Coptic Christians, and despite Sadat's efforts to preserve communal peace, there has been a resurgence of ill will and hostile incidents between the two communities that has coincided with the spread of Moslem fundamentalism. The Coptic Church is the native Christian church of Egypt and Ethiopia.

Al Ahram said that documents confiscated from some of the detainees showed there was coordination between religious leaders and political opposition elements seeking to exploit the present sectarian strife and undermine national unity.

This justification is being hotly disputed by the official opposition Socialist Labor Party and other antigovernment elements still free and willing to talk. They maintain that Sadat is using the religious issue as a cover for a crackdown on a growing opposition to his policies, including his peace treaty with Israel and close alliance with the United States.

Today's incident near St. Mark's Cathedral here began after emotional services at the mosques of two arrested sheiks whose followers were agitated by calls for their release and in one case veiled attacks on the Christian community.

At the mosque of Sheik Mahmoud Kiskh, which was packed for the noontime prayer service of the Moslem holy day, his supporters chanted religious slogans attacking Sadat, while a state-appointed substitute sheik tried to calm the crowd.

At the Mosque of the Light, however, a Moslem fundamentalist preacher, Sheik Mohammed Ghazali, called back the throng leaving after the regular service and proceeded to work them into a religious frenzy by having them repeat a traditional chant of oppressed Moslems, "Allah is our supporter and we rely upon him."

At that point, the crowd from Kiskh's mosque arrived and the two groups marched toward the cathedral.