MANILA, Philippines--Four people were killed stealing ballot boxes, the government said, and a military alert was extended as millions of Filipinos voted this morning for a president--Ferdinand E. Marcos, ruler for 20 years, or Corazon Aquino, widow of his assassinated rival.

A military "red alert" due to run out at noon here was extended indefinitely to deal with the violence and cheating.

Even before the polls opened at 7 a.m., both sides reported fraud and harassment. There were stories of men brandishing rifles and firing in the air to scare waiting voters, stolen ballot boxes, and votes being bought for as little as $1.

"It's a mess, fraud all over," said Ching Montinola, a press spokesman at Aquino headquarters.

Reporting one of the worst incidents, the official Philippine News Agency said communist rebels ambushed an army tank escorting election officials carrying ballot boxes on a road in northern Mindanao, 500 miles south of Manila. The agency report said four attackers were killed and several wounded when the tank returned fire. There were no other casualties, the report said.

In Albay province, a pollwatcher reported from Luzon that voting in four towns abruptly stopped after one hour when unidentified men seized seven ballot boxes.

Election officials said peak voting in the eight-hour voting day normally occurs in the early afternoon, but unusually large numbers of voters were standing in line before the schoolhouse polls opened. Millions walked, rode in ox carts or took ferry boats tocast ballots in one of the most importantelections in Philippine history.

The 26 million registered voters are requiredby law to cast ballots in this nation of 7100islands and 54 million people. Results were notexpected before the weekend.

Senior military officials, interviewed ongovernment television, played down reports ofviolence and said things were under control.

But Gen. Fabian C. Ver., the armed forcescommander, told a television interviewer that a24-hour "red alert" declared at noon yesterday hadbeen extended "particularly in the hot spots toavert any potential election irregularity."

Under the "red alert," all 200,000 members ofthe military were called into the highest state ofreadiness, which is customary for elections here.But Aquino's supporters said they feared soldierswould be used to influence the vote.

Radio reports said a policeman was wounded inBacolod, 300 miles south of Manila. In thecapital, workers for Aquino's UNIDO party chargedthat armed men from Marcos' New Society Movementabducted and beat an Aquino supporter, whosewhereabouts were unknown.

Forty minutes after the polls opened, Aquinocast her vote at a schoolhouse at HaciendaLuisita, a sugar plantation owned by her family inTarlac province, 70 miles north of Manila.Barricades were set up on roads leading to herhometown.

"Today is my day, and I've never been moreconfident of anything in my life as today," saidAquino, 53, whose running mate is Sen. SalvadorLaurel.

Marcos, 68, flew by helicopter to his hometownof Batac, 250 miles north of the capital, to casthis ballot. He said a final survey showed he wouldwin with 63 percent of the vote.