Philippine President Ferdinand Marcos announced today that he will run alone on his party's ticket in a "snap" presidential election on Jan. 17 and that the post of vice president will not be contested.

"Frankly, it should be me alone because the issue is Marcos. This election should be on a one-on-one basis. Therefore, there should be no vice president in these snap elections."

Opposition leaders said they were ready to challenge Marcos, but they said they were concerned about the constitutionality of an early election because of Marcos' refusal to resign.

Marcos' current six-year term ends officially in 1987, when elections for president and vice president were scheduled to take place.

But in an announcement on U.S. television Sunday, Marcos said he would seek an early election, asking the National Assembly to approve an amendment to the election code -- which already provides for special elections in case a president dies, resigns, is disabled or removed from office -- allowing him to "bring a fundamental issue to the people." Government officials later said Marcos would thus remain as president and run as a candidate without resigning.

The National Unification Committee, an umbrella group of all opposition presidential aspirants, said after an emergency meeting today: "The opposition will participate in a snap election for president and vice president, and the opposition will present a single slate."

Among those present in the meeting was the widow of slain opposition leader Benigno Aquino. Corazon Aquino, 52, has emerged as the most likely candidate behind whom the opposition could unite.

Salvador Laurel, another prominent opposition candidate, said in Los Angeles: "We are ready to fight Marcos or any ruling party candidate . . . . We hope this time Mr. Marcos means it."

But Francisco Rodrigo, vice chairman of the umbrella committee and of the United Nationalist Democratic Organization, said, "This is absolutely unconstitutional. He has undue advantage if he remains as president with all the provisions of his power."

Arturo Tolentino, an expert on the constitution whom Marcos fired as foreign minister in March because of his independent views, said there was no provision for an election before the end of Marcos' term unless the post were vacated.

Marcos said he will consult his ruling New Society Party in a caucus on Sunday on legislative requirements. The official Philippine News Agency said the revised code is expected to be passed by the assembly, where Marcos holds a heavy majority, by mid-month.

It was unclear, however, what would happen to elections for a vice president. The succession issue has been of concern here because of Marcos' reported health problems. The post is vacant, and the speaker of the National Assembly is next in succession.

Marcos made today's announcement in Tarlac, 70 miles north of Manila, where he had gone to inspect damage from a recent typhoon.

Opposition and diplomatic observers saw yesterday's surprise announcement on television as a response to increasing concern and criticism in Washington concerning Marcos' ability to make sweeping economic and political changes in the face of an escalating Communist insurgency.

In Washington, a State Department spokesman emphasized U.S. concern for free and fair elections. He said confidence in the electoral process requires professional conduct from the Philippine military, a fair election law, "a truly impartial commission on elections and an accredited, independent citizens' election-monitoring organization."

Members of the opposition said Marcos threatened to call snap elections in August but then reversed himself.