Prime Minister Sirimavo Bandaranaike's bid to remain the world's only woman head of government suffered a serious blow early today in initial returns from Sri Lanka's national parlimentary elections.

The opposition United National Party swept 13 of the first 15 seats declared for the 168 member Parliment and the other two went to a minority party.

The official resul*st, announced over the state radio, indicated broad support for the opposition party virtually all across this Indian Ocean island.

Unofficial reports indicated that Bandranaike, 61, had taken a 5,000 vote lead in her home district, but that some of her Cabinet ministers were trailing in their districts. Observers had predicted defeat for Bandaranaike's Sri Lanka Freedom Party.

Some 6.6 million of 14 million residents were eligible to vote in the country's eighth parlimentary election since independence in 1948.

Reports from across the island, formerly called Ceylon. Indicated that there were only a few minor scuffles. There had been fears of a new out-break of the violence that has mtrred some previous elections. At least 13 persons died in violent incidents during the six-weekcampaign.

The United Left Front - made up of the Communist Party and other leftist groups - was not expected to pose any serious threat to the two main rivals.

The Tamil United Liberation Front seemed assured of winning a parliamentary bloc of between 20 and 25 seats in the predominantly Tamil areas on the east coast.

Its main electoral plank was a demand for creation of a separate Tamil state and its leader said they would not support any of the national parties in the formation of a government.

United National Party leader Junius R. Jayewardene, 17, said none of Sri Lanka's political parties had responded to his election eve invitation to join a "grand coalition" government, if he is elected. He said he was not certain if he would invite Bandaranaike into his Cabinet if he wins.

Jayewardene, a retired lawyer and veteran politician, mounted an agressive campaign against the prime minister's seven-year rule.

He accused her of undermining democratic freedoms with a six-year national emergency and a four-year campaign to muffle Sri Lanka's lively, partisan press.

Jayewardene also accused Bandaranaike of mismanaging the economy and allowing unemployment and inflation to rage unchecked.

Bandaranaike who served an earlier five-year term as prime minister, kept up a confident public face through out the campaign.

Her controversial son Anura, a 28-year-old political power broker, was expected to win the parliament seat he is seeking.

Bandaranaike's opponents compared Anura to Sanjay Gandhi, the once politically poweful son of India's former prime minister Indira Gandhi. She was ousted in March elections by voters upset with the long national emergency she declared and reports of improper activities by her son.

Sri Lankans also find other comparisions between thee two woman leaders.

"We were put under a national emergency in 1971," one disgruntled islander said recently. "We've had curbs on the press for years. We don't like it any more than the Indians.