Returning congressional leaders easily agreed yesterday on a scathing denunciation of the Soviet Union, and also said Congress would pass--though perhaps not so easily--legislation approving continued deployment of U.S. troops in Lebanon.

As the Capitol reopened for business after a five-week summer recess, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and Minority Leader Robert C. Byrd (D-W.Va.) jointly introduced a resolution condemning the "brutal behavior" of the Soviets in shooting down Korean Air Lines Flight 007 the week before last. A similar measure is expected to be quickly introduced in the House, where 23 members took the floor yesterday to denounce the Soviet action.

Meanwhile, Baker and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said they expect congressional action that will permit President Reagan to keep U.S. forces in Lebanon--even though, as O'Neill noted, many members have been urged by their constituents to bring the Marines home.

In other developments as the two houses reconvened:

* O'Neill said the Democrats will not move until the Republicans do to enact the $12 billion tax increase called for in the congressional budget resolution for next fiscal year.

"Any new tax bill will have to come from the president . . . or the Republican Party," he said. Reagan has opposed any tax increase until fiscal 1985.

* The Senate resumed consideration of the administration's proposal to create a Radio Marti to broadcast to Cuba, including a possible compromise to put it under Voice of America. Baker said final Senate action on the measure, which has been bogged down in a filibuster, is possible today.

* Two new members were sworn into the 98th Congress. In the House, Democrat Charles A. Hayes, 65, a former union official, succeeded Harold Washington, now mayor of Chicago. In the Senate, Daniel J. Evans, 57, a former governor of Washington, took the seat to which he was appointed after the death of Henry M. Jackson. The appointment is pending an election this fall. The replacement of Democrat Jackson by Republican Evans increases the GOP's Senate majority to 55 to 45.

The Senate's proposed resolution to condemn the Soviets is stronger in rhetoric than sanctions, which led conservative Republicans to maneuver to make sure it can be amended when it comes to the floor, perhaps Wednesday. When Baker tried to get unanimous consent to bar amendments to the legislation, he was blocked by Sen. William L. Armstrong (R-Colo.), who, along with Sens. Steven D. Symms (R-Idaho), Jesse Helms (R-N.C.) and perhaps others, plans to introduce amendments to "put teeth" in the resolution, one aide said.

The Baker-Byrd resolution condemns the Soviet Union for what it calls "this barbaric action . . . this murderous act . . . this cold-blooded attack . . . one of the most infamous and reprehensible acts of aviation history." In terms of specific action, it affirms steps already taken by Reagan and "urges our allies to cooperate with the U.S. in implementing additional sanctions until . . . the Soviet Union apologizes unequivocally for its actions, fully compensates the families of innocent victims and agrees to abide by internationally recognized and established procedures which are purposefully designed to prevent the occurrence of such tragedies."

It does not spell out what these sanctions might be.

In introducing the resolution, Baker observed that "the civilized world is simply not equipped to deal or even cope with this act of senseless barbarity" and added: "Perhaps it is a measure of hope that we have become too civilized to respond adequately or in kind to events of this nature."

In the House, O'Neill contended that President Carter had taken stronger actions against the Soviets, but said he generally agreed with what Reagan has done. Asked if he thought Reagan should have taken stronger action, O'Neill asked impatiently, "Like what . . . , like what?"

An aide to Armstrong said that, along with more forceful sanctions still being drafted, the Colorado senator is considering separate sections on human rights violations in the Soviet Union and on possible connections between the downing of the Korean jetliner and missile testing by the Soviets.

Leaders in both houses suggested yesterday that Congress will authorize continued deployment of U.S. Marines as part of the multinational peace-keeping force there. There was disagreement, however, about the possible terms of such a measure.

One question concerns application of the 1973 War Powers Resolution, which requires congressional approval of any commitment of U.S. troops facing "hostilities" or "imminent hostilities" overseas. The White House has asked for approval of continued deployment without invoking that law. But many in Congress want it invoked to establish congressional prerogative in this area.

Sen. Charles McC. Mathias Jr. (R-Md.) yesterday introduced a resolution that would invoke the War Powers Resolution but permit the president to keep Marines in Lebanon for six more months. The Mathias resolution would prohibit, however, any expansion of the 1,200-man force or its mission without specific congressional approval.

Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) said he agreed with the "thrust" of Mathias' proposal but not with the six-month restriction.

The White House, meanwhile, sent Reagan adviser James A. Baker III to the Capitol yesterday to confer with O'Neill on the issue. As O'Neill reported it, Baker asked for a statement of congressional support for the mission that would not invoke the War Powers Resolution and would not place limits on Reagan.

The deaths of four of the Marines under hostile shellfire prompted considerable public sentiment to withdraw the force, members said yesterday. Mathias said a constituent, noticing his senatorial license plate, pulled up alongside his car last week and shouted, "Senator, bring our boys home."

"Nobody's telling us to stay over there," O'Neill agreed.

There was some agreement with this view in Congress. Sen. Christopher J. Dodd (D-Conn.) told reporters yesterday that, although it makes him somewhat uncomfortable, he would vote to bring the Marines home. "We look weak if we run, but foolish if we stay," said Dodd.

But leaders of both parties said they expect some resolution authorizing extended deployment to pass both houses, probably early next month. Majority Leader Baker, who criticized the decision to send the Marines to Lebanon, said he thought it would be worse to withdraw them under present conditions.