Congress, led by the Republican-controlled Senate, voted yesterday to block the Reagan administration's Office of Personnel Management from issuing controversial performance standards for civil service pay raises, promotions and layoffs until Congress can consider alternative proposals.

The delay, which would last until Oct. 15, a month after Congress returns from its summer recess, is aimed at a modified version of even more drastic merit proposals from OPM that set off a tempest within both Congress and the civil service earlier this year.

The Senate approved the delay by a vote of 75 to 18, the House by voice vote. The legislation to which it was attached, a compromise $10.9 billion transportation appropriations bill for next year, now goes to President Reagan.

The president had previously indicated that he would sign the transportation bill. But a spokesman for the Office of Management and Budget noted that the administration had objected "terribly strongly" to earlier moves by Congress to throw roadblocks in the way of OPM's plans, and said he could not say if Reagan would now sign the measure.

With the backing of the Washington area's three other senators as well as Senate Republican leaders, Sen. Charles McC. Mathias (R-Md.) pushed for the delay on grounds that it was needed to prevent OPM from implementing its plans before Congress returns from its recess.

One proposed compromise, advanced by Senate Republican Whip Ted Stevens of Alaska, would try out the new performance system on 10 percent of the work force before it can be imposed government-wide.

The proposed OPM rules would allow a worker's performance to be evaluated along with seniority in determining pay, promotions and layoffs. The modifications approved last month address some objections to the earlier plan, such as the length of evaluation periods and appeal rights, but the size of the Senate vote indicated that serious misgivings persist.

Congress tried to block the OPM rules by language in a supplemental appropriations bill for this year that was enacted last week. But it turned out to affect only the earlier OPM proposal, which was superseded by last month's revisions.

By issuing the revised rules in mid-July, after learning of the congressional opposition, OPM was trying to "circumvent the clear intent of Congress," Sen. Paul S. Sarbanes (D-Md.) charged.

But Sen. Steve Symms (R-Idaho) contended that the changes were "modest" and said the main opposition came from four senators who were simply responding to constituent pressure.

Wrestling inconclusively yesterday with several other issues holding up the start of its recess, the Senate broke logjams on legislation for Radio Marti and the default-plagued Washington Public Power Supply System, only to run into further problems on both bills.

On Radio Marti, which the administration wants to set up to broadcast into Cuba, the Senate voted, 61 to 34, to break a filibuster. But the bill faced further delays from opponents, and sources said it would probably be held over.

On WPPSS, Sen. James A. McClure (R-Idaho) won a critical procedural vote to allow consideration of borrowing authority for the Bonneville Power Administration to expedite completion of two WPPSS nuclear power plants not involved in the recent bond default for two other plants.

The WPPSS proposal is part of a $7.6 billion Interior Department appropriations bill that Republican leaders seem determined to pass before the recess starts, theoretically tonight. But senators opposing the WPPSS provision vowed to keep talking against it until the recess starts, if necessary, in order to block its approval.

McClure won his procedural victory, 50 to 47, after Senate Appropriations Committee Chairman Mark O. Hatfield (R-Ore.) came to the rescue by contending it was proper to handle a project on an appropriations bill because Bonneville Power projects are normally handled that way.