Congressional leaders yesterday greeted President Reagan's announcement that he will shortly join them in "productive and constructive" deficit-reduction negotiations with "everything on the table" as a significant step towards resolving the nation's budget impasse and calming volatile financial markets.

Senate Budget Committee Chairman Lawton Chiles (D-Fla.), who has been calling for such a high-level meeting since the beginning of the year, said the president had offered the Congress "a positive opportunity and we ought to move into it."

"Definitely what he said was different than what we have been hearing for the last nine months," said Chiles, in reference to Reagan's new willingness to discuss higher taxes that he had repeatedly pledged to veto. "Events have materially changed. The president and Congress need to try to put this thing together," he said.

House Democratic Whip Tony Coelho (D-Calif.) said of the president's news conference, "If you listened very carefully, you could hear the sounds of cement cracking." The reference was to a remark made by Reagan while governor of California when he finally consented to a state tax increase after previously vowing that his feet were set in cement on the issue.

"I was encouraged by what he said," said Senate Majority Leader Robert C. Byrd (D-W.Va.), hailing what he called Reagan's "considerable courage and statesmanship" in backing off his vows never to consider higher taxes. The change, said Byrd, "indicated to me . . . that he sincerely wants to negotiate and achieve a package we can all support."

Democrats largely chose to ignore Reagan's remarks blaming high deficits on Democratic control of Congress. "At this point I think it is very important we put aside the finger-pointing," said Byrd.

Republican leaders in Congress expressed similar relief at the president's more conciliatory attitude.

House Minority Leader Robert H. Michel (R-Ill.) said, "I take some heart from it; he's been so inflexible. In the final analysis, there has to be some give-and-take between the legislative and executive branches."

Michel, who unlike some of his GOP colleagues has been willing to consider higher taxes, said "it really isn't all that big a deal" to find a mutually acceptable package of spending reductions and new revenues that will meet this year's goal of reducing the deficit by $23 billion.

The taxes necessary to meet the target, said Michel, "are peanuts."

Reagan's announcement at his news conference followed a similar written statement released in mid-afternoon by the White House that Senate Minority Leader Robert J. Dole (R-Kan.) said was "evidence of presidential leadership -- just what we need to reassure the American people."

Sen. Pete V. Domenici (N.M.), the ranking Republican on the Senate Budget Committee, called the president's announcement "about as strong a signal as we {are} going to get that the partisan bickering is behind us . . . . Nothing, in my opinion, can be sacrosanct."

House Speaker Jim Wright (D-Tex.), meanwhile, said yesterday morning that the House would proceed with plans to take up the $12 billion tax increase legislation next week. To put it on hold, as requested by House Republicans, said Wright, "would show hesitation and vacillation."

Wright said that in preliminary discussions Wednesday with top administration officials, both sides had agreed that any deficit package that emerges from the talks would combine equal amounts of new revenues and spending reductions. The speaker said that sales of federal assets, now prohibited under the balanced budget law, may be part of such a package.

Republican members of the House, who have largely backed Reagan in opposing higher taxes, remain sharply divided, however.

Rep. Silvio O. Conte (Mass.), the ranking Republican on the House Appropriations Committee, said that Republicans like himself who favor higher revenues are viewed as "Benedict Arnolds" by many of their GOP colleagues.

"I think he {Reagan} realizes the country is in trouble and he has to give," Conte said. "He's played his hand long enough and it's time to compromise, but he'll get a lot of heat from the right."

Staff writer Edward Walsh contributed to this report.