Montgomery County Council members, in a rare rejection of a labor contract negotiated with a government employe union, derailed a new agreement between County Executive Charles W. Gilchrist and 636 police officers last night and ordered the two sides to resume negotiations.

The council's 4-to-3 vote against a key early-retirement provision of the proposed three-year contract makes the entire pact open to renegotiation. The action stunned a crowd of more than 100 police officers and their family members, who angrily walked out of the council's hearing room in Rockville after the vote.

A small number of the officers who sat through the three-hour voting session last night shouted boos and threw sheaves of papers after the vote.

"For the first time, the council is rejecting an agreement negotiated in good faith," said council member R. Scott Fosler, who voted for the contract along with Council President Michael L. Gudis and Rose Crenca.

The four-member majority argued that the contract was fatally flawed by a provision that would allow 20-year veterans of the Montgomery police force to retire next year and receive a pension equal to 40 percent of their regular pay. Under the current contract, officers retiring after 20 years -- before the traditional 25-year mark -- receive only a 32 percent pension.

Spokesmen for Gilchrist and negotiators for the police officers' union argued that allowing the one-time "window" for early retirements in 1986 would be a bargain for the county government. Highly paid officers earning upwards of $35,000 would leave, making room at the bottom of the pay scale for new recruits, who earn less than $21,000 after their first year on the job, they said.

But opponents of the retirement "window" said it was a dangerous precedent that would lead other public safety unions to demand similarly generous retirements.

"It doesn't by itself break the bank, but the precedent might," said council member David L. Scull.

Council members raised few objections about other provisions of the contract, which would give officers cost-of-living raises totalling 14 percent for the next three years. Extra benefits, including those in a new pay schedule contained in the contract, would give Montgomery police officers an average 8.3 percent raise in the coming year.

The typical Montgomery officer earns about $31,000 annually.

Council members have given themselves an informal deadline of April 25 to tentatively approve the police contract and have a legal deadline of May 15 to approve it as part of the county's budget for next year.