The heat and shock wave of a nuclear blast over the U.S. Capitol building would kill 98 percent of the people in the District of Columbia instantly and kill or injure 50 percent of those in the suburbs as far as 19 miles away, a physician told an audience of 75 who gathered in a Bethesda church last night.

"It's hard to imagine the effects of a nuclear blast," Dr. Wesley Mason of Kensington said at a meeting of the Nuclear Arms Freeze Task Force of Maryland. "One out of 10 physicians would be killed. Of the survivors, there would be one doctor for every 1,700 seriously injured people. Most of the injured would die without ever seeing the doctor."

Mason was joined by Stan Norris, a research analyst for the Center for Defense Information, and Peter Bloch, an administrative law judge for the Nuclear Regulatory Commission, in a forum that explored the Reagan administration's defense policies, the effects of nuclear war and what citizens can do to avert a nuclear holocaust.

"The Reagan defense policies rest on nostalgia--a yearning for earlier times when the U.S. was 'Number One,' " Norris told the group gathered in a chapel at the Cedar Lane Unitarian Church. "There is more and more talk designed to make us think nuclear wars are survivable. But both the U.S. and the Soviets have the capacity to destroy each other, and in many different ways."

Bloch, speaking as an individual, said nationwide efforts in the last year in support of a bilateral freeze on nuclear arms have been an overwhelming success.

"But we have been so successful with this movement," he said, "that already the Reagan administration opposes us and the Soviets have made an offer so dangerously close to our position that we might be confused with Soviet sympathizers."

The Reagan administration has opposed congressional calls for the freeze, saying it would put the U.S. "into a position of military disadvantage and dangerous vulnerability."

The Maryland group asked those who attended last night's forum to sign and circulate petitions urging Maryland's congressional delegation to support a resolution calling for a permanent freeze on nuclear weapons by the U.S. and the Soviet Union.

Citizens at 257 town meetings in five New England states--Vermont, Massachusetts, New Hampshire, Connecticut, and Maine--have passed resolutions endorsing such a freeze. Twenty-three city councils have passed such resolutions, and seven county councils have backed a freeze, including those of Montgomery and Howard counties in Maryland and Loudoun County in Virginia.

Both houses of legislatures in Connecticut, Maine, Massachusetts, Minnesota, Oregon and Vermont have approved resolutions favoring a freeze, and similar resolutions have been introduced in the legislatures of Maryland, Ohio and the state of Washington.

In the District of Columbia, a citizen has asked the Board of Elections and Ethics to consider placing a question on the November ballot giving voters the opportunity to back a nuclear weapons freeze. The board is to decide early next month whether such a question is appropriate; if it decides it is, backers of the move would have to collect 14,000 signatures before the question could be placed on the ballot.