Former president Hernan Siles Zuazo was leading today in unoffical returns from Sunday's presidential election, but since he did not recieve an absolute majority the election should be decided by Bolivia's newly elected Congress.

Under the Bolivian constitution, Congress is to meet August 2 to choose among Siles and the two other frontrunners - former presidents Victor Paz Estenssoro and Gen. Hugo Banzer.

It was generally expected here that one of the two top vote getters - Siles, a leftist, or Paz, a centrist, will be selected as the country's first democratically chosen president after more than a decade of military rule. Banzero, a rightist, served as military ruler for seven years until he was toppled a year ago.

With 63 percent of the vote unofficially tabulated this afternoon, Siles and his Popular and Democratic Union party were ahead with 37 percent of the 1.1 million votes counted.

Paz and his National Revolutionary Movement Alliance were in second place with 29 percent, while Banzer was running a distant third with about 18 percent. It was estimated that somewhere between 1.5 million and 1.6 million valid votes were cast by the country's 1.8 million registered voters.

Although Siles was clearly ahead in the returns tabulated so far, the votes yet to be counted are almost entirely from rural areas, where Paz expects do well among peasants who remember him as the president who broke up Bolivia's large estates to give land to the poor.

Whether Paz will significantly improve his percentage of the vote by winning in the countryside is open to some doubt, according to observers, because Siles did well in rural areas in last year's aborted general election and was far better organized and financed during this year's campaign.

Banzer, whose support came principally from middle and upper middle class voters in the three main cities - La Paz, Santa Cruz, and Cochabamba, - is not expected to do well in the rural areas. His percentage of the total vote could decrease when the final and official returns are announced later this month.

For either Siles or Paz to govern effectively - and avoid bloodshed in this historically volatile country - one will have to agree to a coalition.

Most observers here think that whichever wins a plurality of the popular vote will demand the presidency. Based on the partial returns available today, however, it appeared that while Siles could win a plurality of the votes cast in the election, Paz, party could win more seats in the Congress.

There has been no indication so far that any faction of Bolivia's armed forces is preparing a coup to abort the election, as happened last year when it appeared that Gen. Juan Pereda, the government's official candidate, had not won. The coup came 10 days after the election.