A radical Social Democrat who favors West German withdrawal from NATO's military command was swept to power in the Saarland today in regional elections.

In other results, the Free Democratic Party scored surprisingly well in the two key regional votes, in West Berlin and the Saarland, reversing a decline that threatened the party's survival as coalition partners with Chancellor Helmut Kohl's Christian Democrats.

The regional votes, occurring two years before the next national election, were considered less a plebiscite on the ruling center-right coalition than a popularity test of two rising political stars.

Oskar Lafontaine, 41, carried his party to an absolute majority in the Saarland for the first time, ousting the center-right coalition there.

In West Berlin, the Christian Democrats, led by Mayor Eberhard Diepgen, 47, whose administration is credited with reversing the divided city's economic decline, lost some support. But gains by the Free Democrats easily reconfirmed control of power for the center-right government there.

The Free Democrats rejoiced over their showing in the two elections, and the party's general secretary, Helmut Haussmann, claimed that the results vindicated his belief that the party had now halted its "march into the abyss."

Political commentators said the results of the two elections were so different that the voting could not be construed as an accurate barometer of national political trends.

The opposition Social Democrats went down to their worst defeat ever in West Berlin, which used to be one of the party's most secure territories. The performance there by former defense minister Hans Apel was seen as another blow to the party's moderates, while Lafontaine's Saarland victory is expected to enhance the clout of its left-wing forces.

Chancellor Kohl, commenting on the results, said, "There is light and there is shadow for the Christian Democratic Union this evening."

Lafontaine's radical views attracted many votes from young people who in other states would support the Greens party. In the Saarland, the Greens won less than 3 percent of the vote and thus failed to reach the 5 percent threshold required to hold seats in national or state assemblies.

On the other hand, the Greens' allies in West Berlin, the Alternative List, did well as expected, increasing their share of the vote to 10.5 percent and gaining four seats.

Lafontaine, a fiery orator and a leading crusader against nuclear weapons, capitalized on widespread discontent among voters with Saarland's depressed steel industry and coal mining.

In the new Saarland state assembly, the Social Democrats will have 26 seats, a net gain of two. The Christian Democrats lost five seats to fall to 20 and the Free Democrats gained one for a total of five.

Lafontaine has called for the withdrawal of all U.S. nuclear weapons from West Germany and wants to employ people in small craft industries that would not damage the environment. His growing influence as a leading advocate of environmental and antinuclear causes is likely to strengthen the faction of Social Democrats seeking to build a national "majority to the left" by capturing the issues and constituency of the Greens.

Party moderates, however, argue that the Social Democrats can only find their way back to power by shifting toward the center, emphasizing commitments to the western alliance and finding an effective strategy to combat unemployment.

In West Berlin, Diepgen's genial popularity as a native Berliner stood in contrast to the "carpetbagger" label pinned on Apel, a Hamburg ally of former chancellor Helmut Schmidt. Apel said tonight that internal party conflicts over his candidacy and his outsider's status were chiefly responsible for the party's loss of 11 seats.

The Christian Democrats dropped eight seats in the Berlin assembly, but the Free Democrats gained two to give the coalition a comfortable 13-seat majority.

"For the coalition, the result is better than I ever expected it would be," Diepgen said. "The most important thing is that we were able to strengthen the position of our governing alliance."