Gunmen seized a French diplomat and a Briton working for the United Nations today in the eighth and ninth kidnapings of foreigners since March 14 in Lebanon.

British journalist Alex Collett, 63, working on a three-month assignment in Lebanon as an information consultant for the U.N. Relief and Works Agency, was seized near Khaldah, just south of Beirut. Collett, accompanied by an Austrian colleague, was approaching Beirut when their car was overtaken by another that forced them to stop, according to agency spokesman John Defrates. The Austrian was not held.

French Cultural Attache Gilles Peyrolles, the fourth French diplomat to be seized in four days, was kidnaped in Tripoli, 50 miles north of here. A group calling itself the Lebanese Armed Revolutionary Factions said in a note delivered to a foreign news agency in Beirut that if "our comrade Abdel Qader Saadi" is not released within 48 hours from a French prison, the group would not be "responsible for guaranteeing Peyrolles' safety."

Saadi, whose real name is Georges Abdullah Ibrahim, was arrested in Lyons last October on charges of using a false passport and associating with known criminals, Reuter news agency quoted security sources as saying in Paris.

The note added that two of the group's members being held in Italy were "unjustly detained" and warned that country of reprisal if they are not released.

Groups holding other American, British and French nationals have said they are doing so for causes touching on the Persian Gulf war or the failure of the United States and its allies to censure Israel for its conduct in southern Lebanon.

On Friday, French Vice Consul Marcel Fontaine was led away at gunpoint as he was buying newspapers near the embassy compound in west Beirut.

The embassy's chief of protocol, Marcel Carton, and his daughter Danielle, a secretary, disappeared after leaving their residence in the Christian sector of the capital on their way to work.

Islamic Jihad, the group claiming the fatal bombings of U.S. and French properties here in 1983-84, also claimed responsibility for kidnaping the French on Friday. An anonymous caller speaking on behalf of that organization said the French were seized because of a barter agreement under which France would supply Mirage planes to Saudi Arabia in exchange for oil. Yesterday, Saudi Arabia, which has supported Iraq in its war with Iran, denied there was such a deal.

Islamic Jihad, Arabic for Islamic holy war, appears to be a front for extremist groups determined to drive westerners away from Lebanon as retaliation for their countries' links with Israel.

Fundamentalist Shiite factions who identify strongly with Iran are believed to be the driving force behind it.

Islamic Jihad claims to be holding five Americans: embassy political officer William Buckley; a Catholic priest, the Rev. Lawrence Jenco; Presbyterian minister Benjamin Weir; librarian Peter Kilburn of the American University of Beirut, and the latest, kidnaped on March 16, Terry A. Anderson, the Beirut bureau chief of The Associated Press.

Anderson's abduction, which came after the kidnaping of two British nationals, Geoffrey Nash and Brian Levick, followed a U.S. veto at the United Nations on March 12 of a resolution condemning Israeli measures against the population of southern Lebanon and a decision by Britain to abstain.

Jenco wrote to his family that he was being held hostage for men being held in Kuwait on charges of a series of bombings on Dec. 12, 1983, against U.S., French and Kuwaiti interests.

Vowing to purge Moslem areas of "spies," Islamic Jihad said recently, "Assuming the profession of a journalist, merchant, industrialist, scientist and religious man will, from now on, be no avail to spies."

A Dutch Roman Catholic priest, the Rev. Nicholas Kluiters, disappeared March 14 in the Bekaa Valley of eastern Lebanon. The number of American and British journalists in Beirut has dwindled to six in the past week.

American professors teaching at the university contacted today said they were not panicking but they avoided circulating off the campus.

A European diplomat residing in West Beirut said none of the 10 members of the European Community was planning to evacuate, despite the stepped-up kidnapings. "Countries not having any prisoners and not involved in the gulf war and having no direct links with the Arab-Israeli conflict have nothing to worry about," he commented. "It definitely seems to be an open season for kidnapings, but an evacuation will only be contemplated, as far as I know, when there is intensive shelling."

In the meantime, the streets of west Beirut have become a hunting ground for groups who think they can dictate the policies of Kuwait, Saudi Arabia and the United States through the detention of innocent people, a daily Beirut newspaper wrote today.