On the third anniversary of the communist takeover of Phnom Penh, an exiled leader and a U.S. congressman charged that a present-day "holocaust" is taking place in Cambodia.

Song Chhang, was minister of information in Cambodia before the communist victory, and Rep. Stephen J. Solarz (D-N.Y.) called in a news conference for increased international condemnation of a reign of terror they said is continuing to take place. Chhang advocated an international conference on the subject as a way to bring outside pressure to bear on Cambodia's leaders.

Noting that exact information is unavailable from the isolated country, Solarz estimated that "at the very least" several hundred thousand persons have been killed or have lost their lives as a result of brutal treatment. Chhang, who now lives in Arlington, said 1 million Cambodians have been "slaughtered" and another million "appear to have perished from disease and starvation."

Executive branch officials said yesterday that a death toll of more than 1 million is believed to be a real possibility, in view of reports from recently escaping refugees that pruges and killings are still taking place.

Solarz also called on the United States to provide refuge for some 15,000 Cambodians reported to be in camps in Thailand and elsewhere in Southeast Asia, and to make it clear that other escapees would be welcome here. About 7,500 Cambodians have been admitted to the United States in the past several years, he said.

"We are looking at the death of the ancient and proud Khmer civilization," Chhang said. He called reports of a recent invasion by Vietnamese forces "the final assault on the crumbling Khmer civilization."

U.S. official sources said they give little credence to charges by Cambodian Premier Pol Pot last week that his forces repelled large-scale invasion by Vietname April 7-10.However, the officials said there are strong indications of Vietnamese attacks last week against Cambodian enclaves inside Vietnamese territory.

In a related development, a Swedish journalist recently in Hanoi reported yesterday that he had been told of continuing border incidents, including tank battles, between Vietnam and China.

Rolf Soderberg, who traveled to Ha Karin Soder, attributed the information with Swedish Foreign Minister to "very well-informed foreigners" in the Vietnamese capital. Sources here said there is no confirmation of the reports, which have been heard in diplomatic circles for some time.