Yielding to heavy pressure from financial markets, Congress and advisers, President Reagan yesterday opened the door to an economic "summit" with Congress and the possibility of tax increases to deal with the federal budget deficit.

Reagan said he was directing White House aides to begin talks with congressional leaders and was "willing to be a participant" if necessary to reach a budget agreement. The president, who up to now has refused to consider even the possibility of a tax increase, said he was "willing to look at whatever proposal" the congressional leaders might make.

But senior officials cast doubt on whether Reagan would walk through the doors he had opened. Meeting with reporters less than a half-hour after Reagan's statement, White House spokesman Marlin Fitzwater said that the president "does not envision" the need for personal participation in a summit or a tax increase. {Related story on Page A11.}

Reagan made his statement yesterday after an emergency meeting with senior advisers that was triggered by Monday's stock market collapse. He met for 40 minutes in the living quarters of his White House residence with Treasury Secretary James A. Baker III, Federal Reserve Board Chairman Alan Greenspan, White House chief of staff Howard H. Baker Jr., deputy chief of staff Kenneth M. Duberstein and Beryl W. Sprinkel, the outgoing chairman of the Council of Economic Advisers.

An official familiar with the discussions said "Reagan was hesitant at first but realized that something must be done."

The president's announcement was greeted with relief by leaders of both parties, who had spent much of the day issuing a nearly united appeal to the president to respond to the financial crisis by negotiating a solution to the prolonged budget impasse. "I welcome the opportunity to do this," said Senate Majority Leader Robert C. Byrd (D-W.Va.). "I welcome this change of attitude on the part of the president and I hope we see the same kind of open-mindedness as we go forward."

Senate Minority Leader Robert J. Dole (R-Kan.), who earlier in the day had called for a demonstration of "leadership" by the president, said the time had come for a "new compact with Congress" to address the federal deficit.

Dole called for an early meeting between congressional leaders and the president "to reassure investors that we are going to deal with this problem." He predicted that such a summit might of necessity force some players "to look at their positions," an apparent reference to Reagan's willingness to at least look at a compromise package that includes higher taxes.

Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.), whose panel has approved a tax increase bill that Reagan had earlier denounced, called for a "cooperative effort where the president and the leadership of Congress have to address this problem and make the hard choices."

Administration sources said Reagan was persuaded by pressure from Republican leaders and economic advisers that it was necessary for him to demonstrate leadership on the deficit issue after Monday's record stock market plunge. One adviser acknowledged that Reagan "was not especially reassuring" in his statement on the day of the crash, when he maintained that "all the economic indicators are solid" and said that many investors were simply cashing in on their stock market profits.

The setting for that statement and the one read by Reagan yesterday were identical, but the White House orchestration of the event and the content of the president's message differed sharply.

Reagan emerged from the meeting with his economic advisers at 5 p.m. through the diplomatic entrance at the South Lawn of the White House. A helicopter was waiting there to take him to Bethesda Naval Medical Center, where Nancy Reagan is recuperating from cancer surgery.

On Monday, the helicopter engines were kept running and Reagan answered a few shouted questions as he boarded. Yesterday, the engines were shut down, and a podium had been placed at the entrance for Reagan to read his statement.

After expressing pleasure in yesterday's stock market surge, the president reviewed consultations with other world leaders and said he was "pleased" by steps taken by the Federal Reserve Board to increase the money supply and with actions of two major banks in returning their prime interest rates to lower levels.

Reagan, who had vowed to accept automatic budget reductions rather than negotiate with Congress on a compromise, then said he was "directing that discussions be undertaken with the bipartisan leadership of the Congress for that purpose." He ended his prepared statement by declaring that the nation's "economic fundamentals" are sound and expressing "great confidence" in the future.

The helicopter engines were then started, but Reagan was peppered with shouted questions, and he lingered to answer some of them. It was in these responses that he indicated reluctant willingness to participate directly in an economic summit and at least listen to proposals for new taxes.

Before he departed, Reagan predicted that there would be no recession unless Americans succumbed to fear and stopped buying consumer items. He also made a familiar partisan point, saying that the Democrats controlled Congress and that "Congress was responsible for the deficit."

The president's annnouncement climaxed a gradual administration retreat from positions taken early in the morning by Fitzwater and Office of Management and Budget Director James C. Miller III, who insisted at back-to-back briefings that Reagan was unyielding on the idea of an economic summit.

The announcement also capped a harried day on Capitol Hill as lawmakers stunned by the events on Wall Street both implored the administration to negotiate and engaged in a new round of partisan skirmishing with Miller over higher taxes.

House Speaker Jim Wright (D-Tex.), calling for an end to "partisan political posturing, finger-pointing and blame-placing," urged Reagan to join Congress in "convening an economic summit without crippling preconditions, to deal with America's long-term structural problems."

However, in an appearance before the House Budget Committee to lay out details of $23 billion in across-the-board budget cuts that began taking effect yesterday, Miller strongly reiterated Reagan's unwillingness to participate in negotiations with Congress if a tax increase is part of the agenda. "If you've got taxes on the table, the president wants them off," Miller said.

At a breakfast meeting with reporters earlier in the day, Miller said Reagan would not consider higher taxes even if Congress capitulates to his demand for further cuts in domestic spending. Miller also said the president does not want the $23 billion in automatic cuts to take effect, an action that would become final next month if Congress and the administration cannot otherwise meet this year's new deficit-reduction goal.

Miller's statement angered Democrats on the budget panel, who promptly voted out legislation that partially meets this year's $23 billion deficit-reduction target by raising $12 billion in taxes and saving $4.7 billion through changes in spending policies.