Rival Philippine opposition leaders Corazon Aquino and Salvador Laurel tonight patched up a split and agreed to field a united ticket headed by Aquino to challenge President Ferdinand Marcos in a special election scheduled for February.

Earlier today, Marcos was officially proclaimed the presidential candidate of his ruling party. He named as his running mate Arturo Tolentino, a persistent critic whom Marcos fired as foreign minister earlier this year. Opposition sources said the choice of Tolentino, 75, a popular and respected legislator from Manila, added strength to Marcos' ticket in the vote-rich capital and was one of the factors that influenced Aquino and Laurel to unite.

Laurel, 57, agreed to withdraw his candidacy for president and run for vice president under Aquino, 52, the politically inexperienced but highly popular widow of assassinated opposition leader Benigno Aquino Jr. In return, Aquino agreed to run as the official presidential candidate of Laurel's political party, the United Nationalist Democratic Organization, known as Unido.

If the election goes ahead as scheduled and is reasonably fair, the unification of the opposition on an Aquino-Laurel ticket could present Marcos with the political fight of his life, according to political analysts.

"I'm very happy and relieved," Aquino said as she left the Commission on Elections with Laurel after filing new certificates of candidacy about an hour before a midnight deadline. Each had filed separate candidacies for president earlier, raising fears of a politically disastrous split in the opposition vote that many analysts believed would hand Marcos an easy victory in his bid for reelection.

"I would have disappointed a lot of people" by running for president, Laurel said in explaining his decision to set aside a long-held ambition and accept the number two spot under Aquino. "Unity is the key to dismantling the Marcos dictatorship, and someone had to make the sacrifice," he said, as supporters broke into cheers.

Marcos, 68, has ruled the Philippines for 20 years. Almost nine were under martial law. He was last elected for a six-year term in June 1981, five months after formally lifting martial law. But faced with U.S. pressure in recent months for wide-ranging political, military and economic reforms to counter a growing Communist insurgency, Marcos last month called an early election, now set for Feb. 7, to seek "a new mandate."

The unification of the opposition also was fueled by the archbishop of Manila, Cardinal Jaime Sin, a frequent critic of the Marcos government, and a flood of appeals from a wide range of supporters urging them to run together.

"It was a deluge," said opposition legislator Homobono Adaza. "It was impossible to resist it."

Laurel told reporters that Cardinal Sin "was just saying for us to think it over very carefully and to think of the greater interest of the country."

Aquino and Laurel "were persuaded to agree" by the cardinal at his residence, said another opposition legislator, Marcelo Fernan. He said Sin already knew at that point that Marcos had chosen Tolentino as his running mate on the ticket of his New Society Movement.

Fernan also said that 40 opposition legislators got together to urge Aquino and Laurel to unite, telling them that if they did not, they should not count on the legislators' support.

Marcos' choice of Tolentino surprised some party delegates. Tolentino has strongly opposed the early election, charging that it is unconstitutional and has suggested that Marcos refrain from running for another term. "To my mind, 20 years is too long," Tolentino said last month in urging Marcos to quit.

Marcos' aides acknowledged that the views of the avowed party "maverick" posed some difficulties but said they were outweighed by his popularity in metropolitan Manila, which has 4 million voters and is considered an opposition stronghold. There are 27 million registered voters in the country. In legislative elections last year, Tolentino was the only candidate of the ruling party to win a seat in the Manila municipality. In the capital region, the opposition swept 16 of 21 seats.

In an acceptance speech, Tolentino noted that he has differed with Marcos "on many important public issues" and indicated he would continue to act as an in-house critic. He said his nomination not only proves Marcos "can tolerate and withstand dissent, but it brings together . . . the majority and the opposition in one ticket."

Tolentino added, "This, to me, is a challenge, and I accept this challenge."

His speech drew the loudest applause in what was otherwise a subdued convention. Despite Marcos' address, delivered in powerful tones by an apparently healthy president, the proceedings seemed to generate little spontaneity or enthusiasm among the 2,000 delegates.

Tolentino later told reporters that he still opposed an election bill that called for the early presidential contest. He has said the bill violates the constitution because an early election can be called only in the event of a presidential vacancy, and Marcos has refused to resign before the vote. The bill has been challenged in the Philippine Supreme Court by 10 opposition petitioners, and a hearing is scheduled for Dec. 17.

Tolentino denied that he was being inconsistent by opposing the election and then agreeing to run as Marcos' vice presidential candidate. But he was evidently at some pains to explain his position.

"If the Supreme Court rules there is nothing wrong constitutionally with the bill, it overrules my own opinion," he said.

Opposition sources said, however, that the choice of Tolentino puts Marcos in a position to be able to call off the election if he perceives he could lose it. Ironically for the opposition, the sources said, the president could cite his foes' objections to the election if the Marcos-appointed court finds it unconstitutional.

Marcos announced his choice of Tolentino following a speech in which he delivered one of his harshest attacks on the political opposition. He charged his opponents with waging a "campaign of calumny and slander" against his government and "the entire Filipino race," said they represented "decadence" and "corruption" and accused them of colluding with Communist insurgents. He did not mention any names or cite any evidence.

The decision for a joint opposition ticket comes after an earlier dispute scuttled the scheduled announcement of an Aquino-Laurel ticket on Sunday and led to recriminations between the two camps that appeared to ensure a three-way race, with the opposition splitting the anti-Marcos vote.

Explaining tonight's about-face, Laurel said, "It means subordinating personal interest to national interest."