The opposition Liberal Party took a commmanding lead yesterday in congressional and civic elections marred by the slaying of nine police officers and ballot-box theft by leftist guerrillas.

The vote was expected to boost Liberal Party presidential candidate Virgilio Barco's chances in the May 25 presidential race and, according to United Press International, was seen as a mortal blow to the presidential campaign of Luis Carlos Galan, 42, a lawyer, economist and founder of New Liberalism -- a leftist Liberal offshoot.

Police said leftist rebels of the Popular Liberation Army attacked a vehicle Sunday in which nine policemen were escorting elections official Alberto Contreras near the village of Cintura, 250 miles north of Bogota.

The guerrillas were reported to have fired on the vehicle, killing the policemen, and then to have burned the ballots and voter lists. Contreras was unharmed.

With 85 percent of the vote counted, the Liberal Party garnered 3 million votes or 48 percent of the total. President Belisario Betancur's Conservative Party received 2.3 million or 37 percent, and the New Liberalism Party won 429,000 or 6.8 percent.

The Patriotic Union, a political movement formed by the Colombian Revolutionary Armed Forces guerrilla group, which signed a 1984 truce with the government, got 90,273 votes or 1.4 percent.