The Senate took the first step last night toward assuring regular paydays for thousands of employees of the Health Education and Welfare and the Labor departments and the District of Columbia.

But its move to enact two so-called "continuing resolutions" to permit continued spending by the three governmental bodies encountered immediate and possibly decisive opposition at the other end of the Capitol.

The Senate actions, approved by voice votes soon after they were recommended by the APpropriations Committee at a hastily called meeting, came with the 1978 fiscal year 12 days old and no budgets adopted. That makes most spending - including that for payrolls - illegal under federal and District laws.

THere has been no public threat that District payrolls may be cut off, and a top city official indicated yesterday that there was no immediate danger. But an estimated 43,000 employees of Labor and HEW in the Washington area, and another 200,000 across the country, can expect drastically reduced paychecks next week.

Yesterday's Senate action faces what could be unwavering opposition in the House, which jealously guards its historic right to originate appropriations bills - including continuing resolutions.

Hearing of the impending action earlier yesterday, Chairman George H.Mahon (D-Tex.) of the House Appropriations Committee asked Senate Majority Leader Robert C. Byrd (D-W.VA.) to block such a step . House Speaker Thomas P. O'Neill (D-Mass.) was present at the meeting, and reportedly sided with Mahon.

The Senate committee action on the Labor-HEW money was taken at the urging of Sen. Warren G. magnuson (D-Wash.), acting chairman of the Appropriations Committee. Sen. Patrick J. Leahy (D-Vt.), chairman of the District Appropriations Subcommittee, successfully sought for similar action for the District, saving any speeding by the city now is clearly illegal.

The approval of the two resolutions followed the failure of a joint Senate-House conference committee to agree upon the D.C. budget and a new House rejection of the labor-HEW bill because of a stalemate oer federal financing of abortions under the Medicare program.

If it is decided that a continuing resolution is needed.Mahon said the House would enact one of its own and send it to the Senate for concurrene.

Lawmakers in both congressional chambers, including Mahon, have predicted that the resolution itself would become mired in the abortion dispute and may fail.

Magnuson said the version of the resolution he offered yesterday seeks to stay free of the abortion dispute, and simply to permit normal departmental operational.

The District budget conferees are scheduled to meet again Monday in another attempt to reach agreement on the major disputed issue, the authorization of a proposed $110 million downtwon convention center.

When yesterday's meeting adjourned after two hours of inconclusive discussion. Leahy offered to meet sooner and at any hour - as early as 5 a.m. and as late as midnight - to reach an early decision on the city budget but Rep. William h. Natcher (D-DY.) refused, citing a heavy House work load.

Leahy opposes the convention center while Natcher, chairman of the House District Appropriations Sub-committee, favors it.

They are trying to work out differences between the House version of the city budget, which contains $27 million in start-up costs for the project. on a three-block site near Mount Vernon Square, and the Senate version, which omits all center funds.

Natcher said yesterday he would move to reduce the project by $19.6 million, the cost of the site, in hope a government-owned or privately donated location may be provided. Leahy rejected the proposal, but suggested that lawmakers meet with officials and businessmen during the rall congressional recess to seek ways of pumping private money into the center project.

In the session's strongest language Natcher called the Senate's rejection of the center "one of the meanest, shabbiest things in the world," a step backward in the city's efforts to revitalize downtown.

Despite Leahy's statement that the city is spending money illegally for lack of a congressionally enacted budget, there has been no known effort to force a cutoff of vital municipal services.

D.C. budget director Comer S. Copple issued a brief statement late yesterday saying it is clear that Congress intends to maintain city operations, and adding: "All city serives will continue at the level of the fiscal year 1977 operating authority."

This implied that the city expects to meet its $8 million payroll to 16.500 employees next Tuesday.

The Senate move to adopt a continuing resolution was triggered by protests from federal employees and a letter from HEW Secretary Joseph A. Califano warning of serious consequences from a fund cutoff.

At quitting time yesterday, several federal employees affected by a cutoff expressed hope that the system would somehow reach a solution. Some said they were disgusted with behavior of Congress.

"I'm sure those congressmen are just not aware of the human hardship they're causing," said a Labor Department administrator. "They're sitting over there fat and happy. They're over there talking about abortion and we're over here worrying about living people."

Some employees in the women's bureau of the Labor Department were taking up a "loan collection" of crumpled dollar bills out of their own pockets for their lower paid coworkers.

"We have large numbers of women who are heads of households, who live just above the poverty line," said the bureau's Mildred Crosby. "They're borrowing from us to keep from going to the banks, where they'd have to pay interest, or put up collateral they don't have."