The Portuguese government broadcast appeals for calm today and President Antonio Ramalho Eanes, who is seeking reelection Sunday, stated that the democratic institutions would continue to function following the death yesterday in a plane crash of premier Francisco Sa Carneiro.

Political tension was heightened by round-the-clock contacts between Gen. Eanes and Acting Premier Diogo Freitas do Amaral, by an emergency Cabinet session and by a drawn-out meeting of the national elections council. It determined that the presidential vote should continue as planned.

End-of-campaign rallies due tonight were canceled, however, and thousands visited a monastery -- built in the 16th century to honor the explorers that gave this nation an empire -- where the bodies of Sa Carneiro and defense minister Adelino Amaro da Costa, also killed in the crash, lay in state.

The political unease stems from the acrimony between the dead premier and President Eanes and the power vacuum created by the fatal plane crash.

Heading a coalition known as the Democratic Alliance, Sa Carneiro formed unabashedly right-wing cabinets. His final gamble was an attempt to unseat Earnes by fielding a lackluster opponent.

As opinion polls showed Eanes leading by a margin of up to 20 percent, the premier stepped up his attendances at rallies, criss-crossing Portugal. That commitment led him to board a twin-engined Cessna last evening. It crashed a minute after takeoff from Lisbon Airport. An official inquiry has opened and the government has already ruled out sabotage.

The majority that Sa Carneiro forged promised stability and economic recovery by ridding national politics of the revolutionary excesses that followed the officers' coup six years ago and by cementing close links with the Atlantic Alliance. Portugal is a NATO member and is entering the European Common Market. The premier's death threatens to split the Democratic Alliance coalition, which held together despite internal divisions, thanks to his forceful personality.

Sa Carneiro brooked no rivals and he leaves no apparent heir. Acting Premier Amaral is a member of the junior coalition partner in the ruling majority, the Christian Democratic Party, and has no wide following.

Amid the mourning and bewilderment, speculation centered on the short-term possibility of sectors among Sa Carneiro's Social Democrat followers attempting to form a new majority with the Socialist opposition. Such a lineup has long been favored by President Eanes, who supports a center-left governing majority.

A second possibility was that Amaral would bury the government's current opposition to Eanes in the interests of national unity.

Such speculation and future government majority scenarios depend on the reelection of Eanes Sunday. The death of Sa Carneiro is widely thought to have diminished the already dim chances of rival candidate Gen. Antonio Soares Carneiro. But Portuguese political circles also warned that there could be sympathetic reaction in an emotional electorate that could swing support behind the candidate picked by the dead leader.

As black crepe was hung from balconies in preparation for a state funeral Saturday, the political sources said that the main element in any future instability would rest on the sense of political orphanage felt by an electorate that had voted for a right-wing program enshrined almost totally in the convictions of a man now dead. President Eanes was the mourned leader's principal opponent.