About 75 anti-war activists yesterday protested President Reagan's decision to extend draft registration by marching and chanting peacefully in front of the military recruiting office on Tremont St. in downtown Boston.

Several Harvard students joined the early-afternoon picket line, which was manned by a variety of organizations protesting nuclear power, American intervention in El Salvador, and racism, in addition to the continuation of registration.

"Draft registration is being used as a prop. a tool to build militarism in general in this country," said John Lindsay '82, a member of the Radcliffe-Harvard Peace Alliance.

Members of the Boston-area Central American Solidarity Association and the Boston Draft Resistance Coalition--co-sponsors of yesterday's demonstration--said protests are planned all over the country this week and will continue if the government begins prosecuting non-registrants.

The Justice Department will wait at least a month before seeking prosecutions against the 150 young men the Selective Service has identified as non-registrants, John K. Russell, a department spokesman, said yesterday.

More than 800,000 eligible young men have failed to register since former President Jimmy Carter reinstituted the sign-up program in July 1980, but the Selective Service has not yet begun a formal identification process to provide additional names to law enforcement officials.

Reagan ordered registration last week, reversing his long-standing opposition to the program and reneging on his campaign promise to abolish it.

The president said he had changed his mind after reviewing new statistics showing that registration would save up to six weeks in the event of a prospective mobilization, rather than three to five days, as had previously been assumed.

Administration officials denied reports that the decision was part of an effort to demonstrate American resolve in the wake of the military crackdown in Poland.

Yesterday's protesters sang traditional anti-war songs and chanted familiar slogans such as "No draft, no war; U.S. out of El Salvador."

While some sought warmth in the lobby of 110 Tremont Street, headquarters for military recruiting in Boston, others hung banners from the wrought-iron fence surrounding the historic Granary Burial Ground, resting place of Paul Revere, Samuel Adams and John Hancock, among others.

Midday pedestrians showed only moderate interest in the demonstration, and most seemed more concerned about getting out of the cold. "We've seen this before; it's a good concern, but will it do any good--that's the question," said Samuel P. Cooke, a local office worker