Widespread looting and firebombings in defiance of a nationwide curfew spread across the Sri Lankan capital of Colombo today following the massacre of 35 ethnic Tamil prisoners in their cells, according to Indian news reports here tonight.

In the third day of violence between the Buddhist Sinhalese majority and Hindu Tamils, at least 20 persons were reported killed following the prison massacre. That raises the death toll to more than 100--mostly Tamils--since clashes began Sunday in reprisal for the ambush slaying of 13 Sinhalese soldiers near the Tamil town of Jaffna in the northern part of the island republic.

The international airport at Colombo was closed today, and Colombo-bound flights from Bombay and this southeastern Indian city were canceled. The government of President J.R. Jayewardene continued strict censorship in Sri Lanka.

Tamils, who constitute 20 percent of Sri Lanka's 14.5 million people, have tried in recent years to establish a separate Tamil state in the northern part of the island. The separatist movement, along with differences of religion, race and language between the dark-skinned Tamils and the light-skinned Sinhalese, has long riven Sri Lanka with violence.

The Justice Ministry, in a statement over Sri Lankan radio monitored here, said that the massacre in Colombo's Welikadi Prison occurred when several hundred prisoners attacked Tamils who had been detained under antiterrorism statutes. The dead were said to have included a leader of the Tamil Tigers, a guerrilla group that frequently has clashed with security forces.

Paradoxically, the Tamil underground secessionist movement has intensified during a period when Jayewardene has done more to grant local autonomy than any of his predecessors.

Reuter reported that the Sri Lankan government said about 20,000 Tamils had been made homeless by the rioting. United Press International reported that the government said that "as a result of acts of rioting and thuggery, several civilian deaths have taken place in Jaffna."

Scores of buildings in Colombo reportedly were burned, including the downtown building that houses the State Bank of India and the Indian High Commission. On downtown streets strewn with burning vehicles, heavily armed government troops were reported guarding against further rioting tonight. Of the more than 100 persons killed in the street clashes so far, 75 were said to be Tamils.

India's top diplomat to Sri Lanka, High Commissioner J.S. Chatwal, who returned to Colombo this morning, told reporters here that the homes of several Indian diplomats were attacked, including that of the deputy high commissioner. Dependents of Indian diplomats living in Tamil neighborhoods were evacuated to downtown hotels, according to the United News of India. None of the Indian diplomats was harmed, Reuter reported.

Madras is the capital of the Indian state of Tamil Nadu, from which most of the Sri Lankan Tamils migrated in the 19th century to the former British colony of Ceylon to work on tea and rubber plantations there.

The leader of the Sri Lankan opposition coalition and general secretary of the Tamil United Liberation Front, Appalillai Amirthalingam, issued an appeal to Jayewardene to call in the Indian Army to restore peace, according to Tamil sources here.

Amirthalingam, who has acted as a liaison between the northern Sri Lankan Tamils and Jayewardene's United National Party, cited the precedent of Sri Lanka calling in the Indian Army to help quell a 1971 insurgency, according to Tamil leaders here.

Tamil Nadu's chief minister, M.G. Rama Rao, appealed to Prime Minister Indira Gandhi to raise the situation in Sri Lanka with the United Nations and called for a general strike in this state Aug. 2 to underline the need for action to end communal violence.

Despite the establishment of district development councils in Tamil areas and other efforts by Jayewardene to defuse Tamil tensions, the separatist movement has shown signs of gaining more popular sympathy, which in turn has further alienated the Sinhalese majority.

Agence France-Presse, in a dispatch subject to censorship, reported from Colombo:

As buildings went up in flames, Colombo's looters had a field day in shops and houses, taking away all they could carry. Looters lugging television sets and pedestal fans were seen staggering down the capital's main street.

About 3,000 Tamils who lost their homes and shops in the arson attacks in the Sinhalese-controlled city huddled in schools and religious institutions, Social Service Ministry sources said.