Millions of Filipinos trooped to the polls today amid early reports of fraud in a presidential election widely viewed as the most crucial in Philippine history.

A heavy turnout was apparent at several precincts in Manila and suburbs where nearly 4 million of the country's 26.1 million voters are registered for the contest between President Ferdinand Marcos, 68, a wily politician who has ruled the Philippines for 20 years, and Corazon (Cory) Aquino, 53, widow of a slain opposition leader who has never before run for office.

The stakes for the United States are high. The two military bases here, Clark and Subic, are the largest U.S. overseas bases. There also has been increasing concern within the Reagan administration about Marcos' commitment to major economic and military reforms needed to combat a spreading Communist insurgency and slumping economy that have left the Philippines far behind its neighbors in Southeast Asia.

Amid tensions that appear to be peaking after a bitter two-month campaign, the armed forces yesterday went on "red alert" throughout the country with an officially announced mission to ensure orderly and peaceful voting. Opposition spokesmen reported activities by armed groups in several parts of the country allegedly involving preparations for cheating and intimidation of voters.

Polls opened at 7 a.m. (6 p.m. yesterday EST) and close at 3 p.m. (2 a.m. Friday EST) in 86,023 voting places. Voting is by paper ballot, and official election results are proclaimed by the National Assembly. A citizens' watchdog group that is conducting an unofficial "quick count" expects to announce returns as they come in, with up to half the vote expected to be counted by midnight tonight.

The armed forces deputy chief of staff, Lt. Gen. Fidel Ramos, said yesterday that 51 persons had been killed since Dec. 6 in 103 incidents of election-related violence. He said 81,000 soldiers and police had been deployed to safeguard the election, which is subject to disruption in more than 3,000 "hot spots" in 65 of the archipelago's 74 provinces.

Each side is predicting victory in the voting, the first genuine presidential election to be held in the Philippines in nearly 17 years. The election comes at a time of "unprecedented crisis" in the Philippines and entails both enormous risks and significant opportunities for its democratic institutions, according to a recent report to the U.S. Senate Foreign Relations Committee.

Political turmoil has continued unabated since the 1983 assassination of opposition leader Benigno Aquino Jr., the economy remains mired in its worst crisis since World War II and the Communist insurgency aims to pull the country out of its alliance with the United States and dismantle the two U.S. bases here.

Corazon Aquino, the widow of Benigno Aquino, has blamed these problems on what she calls 20 years of Marcos' pharaoh-like rule and has asserted that her moral leadership will turn the situation around.

Long lines formed in front of polling places inside elementary schools soon after the polls opened. A majority of voters interviewed at random said they had voted for Aquino, who has strong support in the captal.

"Here a majority are for Cory," said a salesman in Manila's teeming slum area of Tondo. Asked why, he answered, "because of poverty." The salesman, who gave his name as Ely, said, however, that one factor with a possible effect on the outcome was vote-buying by Marcos' ruling party.

He said the party sent trucks into the Tondo area last night to distribute two-pound sacks of rice to residents and offer bribes of 30 pesos ($1.66) per vote. Ely said he took the rice but voted for the opposition anyway.

"That's our money," he said. "That's not Marcos' money."

A Marcos organizer in the precinct said he knew nothing about the rice deliveries.

"I didn't get any," he said.

Another voter, Renato Caro, a stevedore who voted at Tondo's Pedro Guevara elementary school, said he cast his ballot for Marcos because, "he is known to us, and he's the one who will solve our problems." He added, "We cannot put a woman as president. A woman is weak."

At the A. Mabini elementary school in Manila's Quiapo district, several voters appeared nervous about acknowledging that they had voted for Aquino, and others refused to discuss the election with reporters.

"People are afraid," said one resident. "Maybe somebody will find out how they voted."

"Even the dead vote here," said a bank employe who gave his name as Rick. He said his brother who has lived in New York since 1979 was listed as having voted in legislative elections in 1984.

One of the early voters in Manila was Imelda Marcos, the wife of the president, who predicted victory for the administration even in the largely pro-opposition capital. She prayed and crossed herself twice as she filled out her ballot and dropped it in the ballot box.

Marcos called the early election more than a year before his current term is due to expire to "seek a new mandate" for his beleaguered leadership. After initially acknowledging that "the issue is Marcos," he began attacking his opponent as a political neophyte who would lead the country of 54 million inhabitants into chaos and communism.

Believing this strategy to have been effective, Marcos' ruling New Society Movement forecasts a comfortable win for the president with 56 percent of the vote. Marcos' running mate is Arturo Tolentino.

The opposition predicts a "landslide" for Aquino with 60 to 70 percent of the vote, but warns that the election could be stolen for Marcos through massive fraud. Aquino is running with former senator Salvador Laurel.

Independent observers agree that much will depend on the ability of Marcos' vaunted political machine, built up in the 1970s while he ruled the country under martial law, to deliver the vote through its control of patronage and the various levers of power.

On the eve of the election the opposition charged that moves were already under way to stuff ballot boxes, switch electoral returns, intimidate voters and thwart the poll-watching activities of an independent citizens' group, the National Citizens Movement for Free Elections, known as Namfrel. The new charges buttressed a statement yesterday by the archbishop of Manila, Cardinal Jaime Sin, that "terrible stories" about the election campaign caused him to wonder whether Filipinos were facing a presidential election or a "contest between good and the forces of evil." Sin called the election "the most crucial in the nation's history."

Marcos issued a statement rejecting that comparison and condemning the opposition for abusing his supporters. The early election is "a choice between alternative programs of government -- not a battle between good and evil," Marcos said.

Opposition officials yesterday said an armed man in plainclothes arrested a low-level Aquino campaign worker at his home in the Manila suburb of Makati on charges of vandalism. Two other campaign workers were also reported to have been arrested, but details were not available.

According to Aquino spokesman Rene Saguisag, incidents have been reported from the provinces in which local officials loyal to Marcos have refused to register opposition poll inspectors, stuffed ballot boxes in advance of the voting and begun buying votes.

Vicente Jayme, vice chairman of Namfrel, said the group had "hard evidence" that Marcos loyalists have distributed carbonless copying paper to voters to place under their ballots and exchange for as much as 200 pesos ($11) for a vote for the Marcos ticket. He said Namfrel also was concerned that some of its estimated 500,000 poll-watchers still had not been accredited by local officials.