West Germany's opposition parties today announced they have formed a coalition and filed a formal motion in parliament for a no-confidence vote Friday to remove Chancellor Helmut Schmidt.

Christian Democratic leader Helmut Kohl, the conservative who is attempting to replace Schmidt as chancellor, announced the go-ahead this evening after deputies from the small Free Democratic Party, formerly allied with Schmidt's Social Democrats, voted 34-to-18 to back the projected new center-right coalition.

"In this very difficult hour for our country, we are prepared together to take on responsibility for the future," said Kohl at a press conference. He was flanked by the other two party leaders in the proposed new government, Free Democratic chief Hans-Dietrich Genscher and Bavaria's Christian Social Union head Franz-Josef Strauss.

"We are aware that we do not have an easy path in front of us. We know what our duty is and we intend to carry it out," he added.

The Free Democrats' caucus result appeared to assure Kohl of a 10- or 11-vote margin of safety in the risky no-confidence maneuver. It was tried, without success, only once before in West Germany.

One Free Democratic deputy from West Berlin, not empowered to vote Friday, was included in today's tensely awaited secret ballot.

In any case, the outcome amounted to at least as many as supported Genscher on Sept. 17 when he pulled his party out of a 13-year partnership with Schmidt's Social Democrats.

The fast swing into an alliance with the conservatives has badly splintered the centrist party, with left-wing members insisting the shift will destroy the party's credibility and drive it out of existence.

The party's executive, in a meeting that preceded the deputies' vote, rejected 18-to-17 a motion demanding that no new coalition be formed before a special party congress called for Oct. 16.

For the unseating of Schmidt to succeed, an absolute majority in the 497-member lower house is required. The Christian Democrats have 174 deputies; their Bavarian sister party, the Christian Social Union, has 52. This leaves a minimum of 23 votes which must come from the Free Democrats.

Because the balloting will be secret, an element of suspense still exists. There is a possibility, although remote, that some Free Democrat or Christian Social deputies could vote to defeat the motion in the interest of embarrassing Kohl or thwarting the plan.

A no-confidence motion brought in 1972 by Rainer Barzel, Kohl's predecessor, to topple Willy Brandt, Schmidt's predecessor, failed by two votes, causing embarrassment to his party and ruining his chances to become chancellor in the future.

If Kohl succeeds, as expected, he and Genscher have promised to call national elections in March. The Free Democrats could fail then to receive the 5 percent needed for representation in parliament. The government to be installed Friday, therefore, could prove short-lived.

Opposition leaders made public a 21-page document negotiated during the past week as the basis for coalition. In foreign affairs, it outlined a basic continuation of existing policy -- a point reassuring for the Free Democrats.

The section on budget and finance policy proposed a number of austerity measures balanced by an increase in value-added taxes and a temporary surtax, to be refunded in the future, on higher incomes.

Talk of a nonrefundable surtax had been a bitter point of contention between the Social Democrats, who wanted one, and the Free Democrats, who did not. That Genscher had ultimately agreed to a variation on such a tax with his new allies drew angry comments from some left-wing Free Democratic deputies.

"We could have had the same with the Social Democrats," said Ingrid Mattheaus-Meier. In a separate vote today on whether to support the proposed coalition program, 32 -- two fewer than backed the no-confidence motion -- said yes.

Expressing the despair of those opposed to the party's swing right, Gerhart Baum, the former interior minister, uttered a variation on an old medical line. "Operation successful, budget reorganized, Free Democratic Party dead," he told reporters.

But the strong backing for Genscher's course from a comfortable majority of the parliamentary caucus suggested that the deputies felt they had no alternative but to proceed with the new coalition. This gives the party until next March to try to rebuild its profile. Schmidt has been pressing for November elections.

The discussion in the caucus lasted eight hours. Up to the end, Social Democrats baited disenchanted Free Democrats with an invitation to reconstruct the old coalition under new Free Democratic leaders.

Schmidt, speaking to his own parliamentary group today, repeated his claim that no mandate existed for the proposed new government. It was also reported that the chancellor plans to give a statement outlining party policies before the vote Friday, as perhaps a combination valedictory and campaign platform.