President Junius R. Jayewardene, who five years ago boldly put then socialist-oriented Sri Lanka on a Western-style free-market economic system, today decisively turned back his leftist challengers in the island nation's first presidential election.

Returns from yesterday's voting by about 8 million Sri Lankans showed Jayewardene polling 53 percent of the vote, defeating Freedom Party candidate Hector Kobbekaduwa, a stand-in for former prime minister Sirimavo Bandaranaike. By getting more than 50 percent, he avoided a complicated second preference tallying system.

Four other leftist parties were eliminated in the election, which in effect was a referendum on whether Jayewardene would be allowed to continue to try to make a capitalist economic system work in a Third World country.

Jayewardene won most of the 22 election districts in the West Virginia-sized country except those with predominantly Tamil -- or south Indian-origin -- population. The Tamils, who constitute about a fifth of the population, boycotted the vote.

Anticipating an outbreak of the kind of violence that has marred previous elections here, Jayewardene last night declared a limited national emergency in which security forces were placed on alert under orders to break up demonstrations.

The Defense Ministry said there had been some disturbances after the polls closed but gave no details.

Small clusters of troops were seen in parts of the capital early today, but there was no curfew or imposition of press censorship as there has been in the three other emergencies declared in the past four years. The government said that the state of emergency, which must be sanctioned by the parliament within 30 days, was a preventive measure.

The election reversed a tradition in Sri Lankan elections, in which voters have turned out of office every incumbent government that has run for reelection.

If Kobbekaduwa had won, he most likely would not have completely dismantled the free-market system, which includes a Singapore-like free-trade zone and other inducements to foreign investment. But he had stirred concern among foreign investors by indicating that he would divert some funds from industrial development in order to restore state subsidies on essential foods and revitalize social programs that flourished under Bandaranaike's mildly socialist government.

Jayewardene's party in 1977 swept away Bandaranaike's Freedom Party government, and he immediately rewrote the constitution, dissolved parliament and elevated himself from prime minister to an executive presidency with a six-year term.