More than six months after President Reagan signed a bill that established a national institute for peace studies, the White House has blocked the agency from organizing or giving out grants.

Knight-Ridder News Service reported that groups in the House and Senate sent letters to Reagan protesting his failure to nominate a board of directors for the U.S. Institute of Peace by April 20, as required by law.

Without a board, none of the $4 million appropriated for the institute can be used for research, training, or grants for peace studies at the nation's universities.

Supporters say the nominations are delayed because of administration resistance to the institute. After opposing the plan, Reagan signed the bill creating the institute when it was attached to a defense authorization.

"If this had been a commission to strengthen the defense budget, there would have been a board of directors nominated very quickly. Now the president is violating a law that he signed himself," said Rep. Dan Glickman (D-Kan.), who signed the protest letter.

A White House spokesman denied that the delay in the nominating the 15 board members was deliberate.

"Any nomination has to go through a process of clearance, and that clearance is not yet complete," said Bob Sims, White House deputy press secretary.