Egyptian authorities announced today the arrest of an unspecified number of Moslem extremists for plotting to seize power and apparently to assassinate President Hosni Mubarak with help from unidentified foreigners.

A statement by the attorney general's office said police had broken up "fanatical clandestine groups" belonging to the Islamic extremist organization Jihad (holy struggle), which was involved in the assassination of president Anwar Sadat last Oct. 6.

The statement said the extremists were planning to storm prisons where Jihad members are being held, to assassinate unidentified persons, and to seize power. While the statement did not specify that those arrested planned to kill Mubarak, it hinted strongly that this was among their objectives.

Although security forces have been rounding up extremists regularly since Sadat's death, this was the first time the government has announced discovery of a plot to seize power and assassinate political leaders since Mubarak took office Oct. 9.

The statement, released by the official Middle East News Agency, said the extremists had also devised an alternative plan to hijack a civilian airliner and then demand the release of imprisoned Jihad members and their safe conduct out of Egypt.

Several thousand suspected members of Jihad and other fundamentalist groups were arrested in the wake of Sadat's assassination. The government freed more than 1,200 this summer and others were released in small numbers last winter. It is not clear how many are still detained, although the number is believed to run into the thousands.

Jihad has emerged as the most active of a number of extremist organizations still operational in Egypt. It was the main group behind the assassination of Sadat, carried out by lieutenant Khalid Islambouli and three others during a military parade. The four and a man identified as the organizer of the plot, Abdel Salam Farag, were executed in March. Seventeen received long prison sentences as accomplices.

Mubarak has been dealing with the Moslem fundamentalists with a mixture of firmness and tolerance, releasing many of them and allowing a new state-supervised Moslem weekly to air their views but also moving quickly to stamp out all clandestine activity.