Tony Benn, an ardent campaigner against capitalism in Britain, today announced that he will attempt to become the Labor Party's deputy leader in Parliament, a move that would increase the power of the militants within the party.

Benn, who has been an architect of the militant socialists' gradual takeover of Britain's ideologically torn opposition party, will try to oust the present deputy leader, Denis Healey.Healey is an outspoken opponent of many of the radical leftist aims that Benn's faction has recently succeeded in having adopted as party policy.

Benn made clear that he is acting to ensure that those policies -- from nationalizing most industry to banning nuclear weapons from Britain -- will be strongly advocated by the party in opposition and enacted into law if Labor wins control of Parliament. p

But other Labor legislators, including some on the left, indicated displeasure with Benn's decision, which they fear will further divide the party during the six months before the deputy leadership election this autumn.

Numerous moderates, including four former Cabinet ministers and 13 members of Parliament, already have left Labor to form a new centrist Social Democratic Party, which has won surprisingly strong support in opinion polls. More than a third of Labor's voters in the last national election are switching their support to the Social Democrats, according to the opinion polls, and appear less likely to return to the fold than disaffected former supporters of Prime Minister Margaret Thatcher's ruling Conservatives.

Labor Party leader Michael Foot tried unsuccessfully to dissuade Benn from challenging Healey. Foot, a less militant leftist than Benn, has been trying to hold the party together since his own compromise selection as leader last November.

He has endorsed a move by the remaining Labor moderates to try to alter what became a symbol of the militant left's takeover of the party's machinery and policy-making: the removal of the leadership selection process from Labor's members of Parliament to an electoral college in which they have only 30 percent of the voting strength compared to 40 percent for the labor unions that finance the party and 30 percent for its local organizations.

But just before the party is to consider this question at its autumn conference, Benn will face Healey and perhaps other candidates in the electoral college and would be counting on his strength among local party organizations and left-wing labor union leaders. Because of Benn's relative lack of popularity in Parliament, this is seen as his best opportunity to gain a leadership position.

Benn, who was energy minister in the last Labor government, wants to remove most industry from capitalist ownership, expand the welfare state, outlaw private medicine and schools, abolish the House of Lords, take Britain out of the Common Market and ban nuclear weapons and foreign bases.

"We must stop America from dragging us into a war by using our bases," he said today.

In his criss-crossing of the country to lecture party workers, labor unions and students, Benn offers a vision of a Britain with worker control of industry, plentiful public services, equal opportunity for women and nonwhite minorities and more democratic decision-making in government, political parties and business.

He opposes both American-style capitalism and Soviet-style "corporate" socialism, contending they are equally incompatible with true democracy.

But Benn, who renounced an inherited title to keep the parliamentary seat he was first elected in 1950, is portrayed by his opponents inside Labor as an ambitious opportunist trying to capitalize on the party's steady decline since the 1960s.

Moderates who have chosen to stay in the party recently organized a new group, called Labor Solidarity, to stop its leftward shift and they appear headed on a collision course with Benn and his followers.