The big guns of the Socialist International came to Ronald Reagan's neoconservative Washington this weekend to give moral support to this country's struggling socialists, and the irony was inescapable.

In a city where even tepid liberalism feels out of favor in the transition maneuvering, socialists from both sides of the Atlantic gathered to discuss the future of democratic socialism in the Western world.

Although welfare has emerged as one of main bugbears of the new American right, Sweden's former prime minister Olof Palme talked of rescuing the welfare state. With Americans clamoring for lower taxes and less public spending, former Dutch prime minister Joop den Uyl underscored the need to maintain higher rates of taxation and government expenditures. While many in this country wonder whether the unions are getting too powerful, Britain's Tony Benn preached about strengthening the labor movement.

Still the irony of the situation failed to keep anyone from attending what is likely to go down in the annals as one of the largest socialist gatherings in this country in decades.

Nearly 2,500 people jammed the opening session Friday night -- and returned Saturday and Sunday -- and had to wait for the conference supervisor, author and activist Michael Harrington, to make about 200 standees leave the room to comply with fire regulations.

"Of all the problems to have," Harrington beamed. "To have, in a United States which just elected Ronald Reagan, too many people at a socialists' meeting."

One of the questions most asked of the European socialist leaders was why so many of them bothered to come to such an assembly in a country where socialism is still considered well out of the political mainstream. In addition to Benn, Palme and Den Uyl, there were such luminaries as West Germany's Willy Brandt, Francois Mitterrand and Michel Rocard of France and Felipe Gonzalez of Spain.

One often-expressed concern among some of the Europeans was that there were no Americans of similar stature -- for instance, liberal Democrats such as Sen. Edward Kennedy, who declined an invitation to attend, reportedly saw several of the socialist leaders at a private dinner Saturday night.

Minneapolis Mayor Donald Fraser took part in a workshop and Rep. Ronald Dellums (D-Calif.) was a keynote speaker.Otherwise, the conference was largely attended by academics, trade unionists and students.

As for the liberal politicians who did not show up, International Association of Machinists President William Winpisinger said, "We can only advise our missing friends, either lead or follow, but get the hell out of the way."

The conference was organized by Harrington's Institute for Democratic Socialism, part of a small democratic socialist movement in the United States that is divided on foreign policy and electoral strategy -- whether to work outside of the Democratic Party or to organize a union-based party such as Britain's Labor Party.

Benn, a leader of the resurgent left wing of that Labor Party, spoke at a press briefing of presenting a "useful reinforcement" to the American socialists -- "a sort of European cavalry arriving at just the right moment."

"We have a vested interest in seeing what the policies of the new U.S. administration will be," Benn said, adding that he thought a Reagan administration likely to follow in the monetarist footsteps of British Prime Minister Margaret Thatcher would encounter serious problems, and that the failure of the new conservatism would lead to fertile ground for the socialist alternative.

Besides stressing solidarity with their socialist brothers and sisters and making socialism a respectable word in political conversations in a country that never fully recovered from the McCarthy era, it was clear that the European Socialists were concerned with continuing their recent commitment to broaden their influence from Europe to the Americas and the Third World.

A recently formed Socialist International committee headed by Gonzalez and including Brandt, Palme and Harrington met privately Saturday to discuss support of leftist parties in Nicaragua, El Salvador and elsewhere in Central America.

The European Socialists have disagreed with the Carter administration about which groups to support in Central America, and some of them will be meeting today with State Department officials to discuss their differences. Bernt Carlsson, the international's general secretary, said yesterday, "We are convinced the U.S. policy is wrong."