President Reagan yesterday hailed the government's announcement that U.S. economic output increased at a robust 8.7 percent annual rate last quarter, calling it a vindication of his economic policies. The economy is "growing with more vigor than most economists predicted," he said.

Reagan acknowledged in a 14-minute news conference that, despite the recovery, the unemployment rate--now 10 percent for civilians--is likely to remain high for many more months, but said that much of the pain from this is concentrated in a small part of the work force.

And he endorsed the moderate new squeeze the Federal Reserve Board has put on the money supply to keep the recovery from getting out of hand and causing a revival of inflation. He also continued to urge Congress to reduce the impending deficit, but by spending cuts rather than by tax increases.

"We support the commitment of the Federal Reserve Board to a monetary policy that ensures stable prices," Reagan told reporters in the White House briefing room, "and we urge the Congress to help us make the Fed's job easier, not by taking more money out of the people's pockets, but by exercising discipline to hold down federal spending."

Earlier in the day Reagan's aides denied that the president has abandoned his support of a $50 billion standby tax proposal for 1986 and thereafter if deficits remain high. They acknowledged that the White House has not been lobbying for the increase.

The reason, they said, is that the president wants to work on spending first, and still believes he can bring it down far enough to make a tax increase unnecessary.

In testimony before the Senate Banking, Finance and Urban Affairs Committee, Martin Feldstein, chairman of the Council of Economic Advisers also fully endorsed the recent Fed actions, but he warned that high interest rates relative to inflation are "significantly reducing the chance of a healthy and balanced recovery."

Such real interest rates are "likely to remain high or rise further" unless a federal budget is adopted that provides "an unambiguous assurance" of declining deficits in the future, Feldstein declared.

In a meeting with reporters, the CEA chairman said that a failure to deal promptly with the budget issue will mean "a more lopsided recovery and a more fragile recovery that may not make it through 1985" and possibly not through 1984.

The administration and Congress will "run a very serious economic risk" if they find no solution, he said.

At the Treasury, assistant secretary for public affairs Ann McLaughlin said, "In the absence of spending cuts, we are not preparing a contingency tax bill. But as a policy recommendation, it's still valid."

McLaughlin also said Treasury Secretary Donald T. Regan believes that the economy is growing strongly enough that "we can get through until '85" without any action on the contingency tax.

But Feldstein stressed that the faster-than-expected recovery will not solve the budget deficit problem. "Despite the recent increase in the projected rate of economic growth, budget deficits are likely to remain in the $150 billion to $200 billion range unless legislative action is taken," he said.

"As long as the Federal Reserve follows a prudent monetary policy that avoids raising the future inflation rate, large out-year deficits will inevitably mean high real interest rates," he added.

Feldstein said that the danger to the recovery from high interest rates could come as early as 1984.

Meanwhile, on the House side of Capitol Hill, Ways and Means Committee members said they were convinced after two days of hearings on possible tax increases that no action is likely in the absence of clear-cut leadership from the administration.

"There is a consensus that something has to be done about revenues," said committee chairman Dan Rostenkowski (D-Ill.). "But we're all political animals here, and I agree that next year that we will be much more political. There will be no appetite for looking at revenues.

"But the entire world is looking at our deficits," Rostenkowski declared. "Without the leadership of the White House, there are very few profiles in courage on Capitol Hill."

Some committee members said that by this fall, if interest rates continue to climb so that consumers and would-be homebuyers are getting pinched again, congressional sentiment against moving on taxes without Reagan's backing could change.

Whether interest rates will go up that much is unclear. CEA Chairman Feldstein would make no precise interest rate forecast, but he said that many financial analysts expect short-term rates to continue upward which "is likely to cause banks to raise their prime lending rate at almost any time." The prime rate has been at 10.5 percent since early March.

Fed Chairman Paul A. Volcker, who preceded Feldstein before the Banking Committee, also appealed for action to reduce deficits. A $50 billion reduction in the $200 billion deficits could reduce interest rates by a percentage point, he said. If financial markets were convinced such a cut would be followed by others, the drop in rates could be larger, he predicted.

After his appearance, the committee voted 16 to 2 to recommend the Senate confirm Volcker for a second four-year term as head of the central bank.

The president, asked if the recovery offers hope for the 11 million unemployed, said: "Oh, yes . . . We're out of that 10 percent bracket now. We look at the unemployed and we are inclined to believe that this is the same group of people that have been sitting there helpless and hopeless throughout the entire recession. Seventy percent of the unemployed today have been unemployed seven weeks or less. Thirty percent of the unemployed today are newcomers to the job market seeking their first employment.

A statistician for the the Bureau of Labor Statistics, Phil Rones, said yesterday Reagan was incorrect on two points: Latest statistics show only 39 percent have been unemployed under seven weeks, and only 13 percent are first-time job hunters, according to government statistics.