About 50 members of the Nuclear Weapons Freeze Campaign, a group opposed to the testing and development of nuclear weapons, released dozens of balloons across the street from the White House yesterday to protest U.S. plans to hold another nuclear warhead test.

The Lafayette Park demonstration was directed at President Reagan, who was accused by protest leaders of ignoring Soviet test ban initiatives, and was staged in support of proposed legislation that would cut off funds for nuclear warhead tests.

"President Ronald Reagan has the chance of a lifetime to avoid a lifetime of chance for his fellow countrypeople and for the human family," said Dr. Jane Gruenebaum, executive director of the Freeze Campaign, reading from a statement.

Gruenebaum said the president should agree to a Soviet proposal to halt explosions of nuclear warheads and to an offer to allow on-site inspections to verify the testing ban.

"At present, the only thing we seem unable to verify is Ronald Reagan's sincerity in achieving that to which he has given lip service -- an end to the nuclear arms race," Gruenebaum said.

The demonstrators, who said they represented freeze movement leaders in 40 states, released clusters of black balloons to symbolize the eight American tests conducted in Nevada since the Soviet moratorium on warhead testing began last August.

White balloons also were released as a sign of hope for an end to the arms race, demonstrators said.

Calling their protest a "trial balloon campaign" to test Reagan's sincerity about ending the threat of nuclear war, organizers said they will urge people to write the president in support of a testing halt and will demonstrate across the country, including the Nevada test site, if new testing occurs.

"The only way to stop the arms race is to stop," Gruenebaum said.