A caption yesterday incorrectly stated the first name of Columbia's president, Cesar Gaviria. (Published 12/11/90)

BOGOTA, COLOMBIA, DEC. 9 -- Leftist ex-rebels won the largest bloc of delegates in today's elections for an assembly charged with rewriting Colombia's constitution.

Leaders across the political spectrum said that extradition, considered by the United States to be an essential tool in combating drug trafficking here, would almost certainly be banned under the new charter.

"There is not much to be optimistic about in the assembly," said a U.S. official. "It could turn out to be a disaster."

With 75 percent of the votes counted, the M-19 Democratic Alliance, which abandoned armed revolution only nine months ago, had received 28 percent of the vote and about 23 seats in the 70-seat assembly under a complex allocation system.

The voting represented the sharpest political realignment in Colombia in this century and a defeat for the fragmented Liberal and Conservative parties, which have dominated national politics for decades. They spent much of the campaign attacking the violent past of the M-19 and its leader Antonio Navarro Wolf. Less than 40 percent of the electorate voted.

A faction of the Conservatives took the second-largest bloc of seats -- about 12 -- with about 16 percent of the vote. The faction's leader, Alvaro Gomez, was kidnapped by the M-19 in 1988 when Navarro was a senior rebel commander.

No group had been expected to win an absolute majority, largely because the Liberals ran 36 separate lists of candidates, and the Conservatives four separate lists, splitting their votes.

In other developments, the military announced that it had attacked and occupied the headquarters of the Marxist Revolutionary Armed Forces of Colombia. Up to 40 guerrillas and 11 soldiers were killed in the fighting in Meta Province, 100 miles southeast of the capital.

President Cesar Gaviria, in a nationally broadcast address Saturday night, said the principal goal of the assembly would be to "face the violence and win that battle definitively." He said judicial reform, "to end the impunity that has led to frustration, hate and violence," would also be a government priority.

When the assembly begins its six-month session in February, it will have unlimited power to change the constitution. It will face issues from revising the Congress to transforming the judicial system. And, while it was hardly mentioned in the campaign because of hostages being held, debate is expected to deal with drug trafficking and violence.

The traffickers, who call themselves the Extraditables, have demanded that the assembly ban extradition and grant them the status of a political movement, which would make them eligible for a blanket amnesty for their crimes, similiar to the one given the M-19 when the guerrillas abandoned their armed struggle.

Foreign and national intelligence analysts say the Extraditables will use eight kidnapped journalists, several from prominent families expected to wield influence in the assembly, as bargaining chips to ensure the assembly carries out the measures, although two may be freed soon as a goodwill gesture.

While granting the traffickers political status will be debated, it is not yet clear what the prospects of that move will be.

"The Extraditables know that until the assembly, the government can only offer certain things, then it hits a legal wall," a senior government official said. "With the assembly, for the first time the limits of the state will not be fixed. They can move the legal wall."

The M-19 was one of the most feared and violent guerrilla groups in Latin America and carried out a series of spectacular raids, including the 1985 takeover of the Supreme Court, in which 109 people died, including 11 of the 24 justices. Now, its leaders say, the rebels have changed.

"If we lead a majority bloc in the assembly, we will transform Colombian politics," said Navarro in an interview shortly before the elections. "We are the only ones that represent change, therefore we are seen by some as a threat. But others see us as the only hope."

Navarro, 41, an engineer who lost a leg in a grenade attack, finished third in May presidential elections. Gaviria, a Liberal, then named him minister of health.