Sri Lanka's Parliament debated a constitutional amendment tonight that effectively would strip the country's 3.5 million ethnic Tamils of elected representation.

Amid extraordinary security and empty public galleries, Parliament debated emergency legislation introduced at the height of week-long violence that has left, by official count, 267 persons--mostly Tamils--dead and thousands of Tamil-owned homes and businesses burned.

The 17 members of the Tamil United Liberation Front, the largest opposition party, boycotted the session, saying they could not be confident that security forces controlled by the majority Sinhalese could protect them.

The front, which has led a campaign for a separate state for Tamils who live mostly in the northern and eastern provinces, charged that Sinhalese soldiers have participated in attacks on Tamils in the the most severe ethnic violence since Sri Lanka became independent in 1948.

Under the terms of the legislation introduced by President Junius R. Jayewardene, whose United National Party holds 80 percent of the seats in Parliament, any discussion of separatism would be prohibited. Stringent punishment would be imposed on violators, including the stripping of all civil rights, confiscation of property and the denial of the right to practice a profession or belong to any organization.

Parties that advocate separatism would be banned under the measure, and Tamil front members who refuse to take an oath not to endorse a division of the country would be refused entry to Parliament.

The Supreme Court ruled the proposal constitutional yesterday, except for the clauses providing for unlimited seizure of property of persons calling for separatism.

Jayewardene outlined for Parliament today details of what he termed a plot by leftist opposition parties to overthrow the government by instigating communal violence. Earlier this week he banned three parties, including the Sri Lanka Communist Party and the Marxist-Leninist People's Liberation Front, and security forces since have been arresting members of both opposition groups.

Jayewardene said the plot was designed to feed on longstanding distrust between the majority Sinhalese and the minority Tamils and create communal violence as a cover for leftist officers of the Army to stage a coup.

As the debate on the amendment opened, Army sharpshooters patrolled the roof of the Parliament building and troops maintained a heavy guard inside and out. Reporters were searched, and a committee appointed by the speaker of the Parliament censored reports of the proceedings.

Despite predictions that the debate today would trigger renewed violence, there were no reports of new incidents in the capital. A government spokesman said authorities in Trincomalee, 200 miles northeast of here, reported that 48 Tamil huts were burned yesterday, but that there were no deaths.

Meanwhile, informed sources here said the Sri Lankan government had asked the United States to provide C130 Hercules transport aircraft so that the Air Force could airlift refugees and supplies to the Tamil city of Jaffna. The Sri Lankans were told, the sources said, that the U.S. government was willing to give supplies to meet humanitarian needs, but did not want to provide assistance that could be interpreted as military aid or give the appearance of interfering in Sinhalese-Tamil troubles.

A senior Sri Lankan official confirmed that Jayewardene also had asked for nonmilitary assistance from Bangladesh and Pakistan.