The most grisly of the photographs depict the remains of the mass graves at Tonle Bati in Cambodia. One shows a pyramid stack of skulls with a line of bones neatly arranged beneath it. Sunlight streams improbably through doorways behind the bones, which are heaped on the floor of what was once a schoolhouse in Cambodia. Another photograph displays rows of skulls like so many white bowling balls lying outside in the brush.

The mildest are the photos of what once was--where there are no remains. There is the destroyed monks' quarters at the Pichey Udong Temple in Kompot Province. Children play nearby.

They all tell the story of a Cambodia brutalized by the reign of Pol Pot from 1975 to 1979. The exhibit, "Cambodia Witness," was organized by Amnesty International and consists of 52 photographs--all but two taken by David Hawk, a former executive director of Amnesty International U.S.A.

The exhibit opened yesterday in the rotunda of the Russell Senate Office Building, which Sen. Christopher Dodd (D-Conn.) secured for the group. He was out of town for the opening, but 150 others showed up, including a sprinkling of senators and congressmen, friends of Hawk, and supporters of human rights such as former Massachusetts congressman the Rev. Robert Drinan.

Maha Ghosanando, a Buddhist monk and a survivor of the Pol Pot regime, was also there. A slight, reserved presence in orange robes, he bowed instead of shaking hands and chanted through part of his remarks to the group.

" . . . It's a sad commentary on our own government that even after the evidence is there, we still manage to ignore the Asian Auschwitz that was Cambodia," Rep. Stephen Solarz (D-N.Y.) told the group which came to see the exhibit. Solarz was instrumental in arranging for the exhibit to continue on display from May 2 to May 13 in the rotunda of the Cannon House Office Building.

An estimated half-million people were executed by the Khmer Rouge during the Pol Pot regime, which lasted until Vietnam invaded Cambodia in 1979. Another 1.5 to 2 million people died from forced labor, starvation or disease.

Many of Hawk's photographs show the prison, Tuol Sleng. "It was not a political prison," said Hawk, who is now lecturing at Hunter College in New York and researching that era of Cambodian history at Columbia University's Center for the Study of Human Rights. "It was an extermination camp--15,000 people were processed to death through this place. There were only seven survivors." He has a picture of one.

Hawk went to Cambodia twice, once in 1981 and once in 1982 to photograph the prison, the destroyed pagodas, the graves. "The mass graves were awful," he said. "You had to visit them early in the morning before the mid-day tropical heat made the stench unbearable."

"It certainly is genocide," said Rose Styron, Amnesty International board member, last night. The organization is working to heighten congressional sensitivity to what happened in Cambodia. Eventually, it would like the Senate to ratify an international agreement against genocide, first proposed by President Truman more than 30 years ago.

"I've been pushing for that treaty for a long time," said Sen. Claiborne Pell (D-R.I.). "We're one of the few countries that haven't ratified it . . . But it's going to require a certain change in thought on the part of my colleagues."