ABIDJAN, IVORY COAST, JUNE 23 -- Police in Nigeria today interrogated opposition leader and self-declared president Moshood Abiola after he was arrested overnight, and hundreds of people reportedly marched through central Lagos tonight to demand his immediate release.

The marchers, mostly young people, chanted as crowds of commuters heading home from work applauded them, according to the Reuter news agency.

Abiola, a millionaire publisher who apparently won a presidential election last year that the military annulled, was arrested at his home after making a public appearance in downtown Lagos, the country's largest city, and declaring that he would announce a rival government within 30 days.

Police have not said where they are holding him, but the Associated Press reported that sources close to Abiola's family said security agents took him to the airport this morning for a flight to Abuja, the capital.

Abiola used his mobile telephone to notify journalists of his arrest, which capped his 11 days as a fugitive from the military regime he opposes.

The United States and Britain, Nigeria's former colonial ruler, condemned his detention, and pro-democracy groups called for street demonstrations. State Department spokesman Mike McCurry said in Washington that Abiola's detention raised serious questions about the military government's commitment to restore democracy.

The military government of Gen. Sani Abacha, who seized power in November from an interim civilian administration installed by Nigeria's former military ruler, Gen. Ibrahim Babangida, has threatened to charge Abiola with treason. The state-run radio said today that Abacha visited troops and urged them to resist "self-proclaimed" democrats.

The report also said Abacha promised salary benefits and new equipment and uniforms -- an apparent attempt to appease poorly paid lower-ranking soldiers who have grown tired of corrupt military rule.

Ironically, Abiola's arrest was the moment many of the country's pro-democracy leaders had been awaiting -- a sign that he had the courage to face the military leaders he criticizes.

Recent attempts to mobilize popular support for Abiola had been ignored, largely because of a feeling of powerlessness after the military canceled last year's elections. Also, many people thought that if Abiola wasn't willing to put himself in danger, why should they, given that scores were killed in last August's rioting.

The anniversary of the annulled elections, June 12, was expected to be the final showdown between Abiola and Abacha. Instead, Abiola declared himself president and went into hiding, and Abacha flew to Tunis for a summit of African leaders.

After piquing the hopes of his supporters several times in the past year, the closest Abiola so far has come to becoming the leader of Africa's most populous nation was when he took Abacha's seat at the inauguration of South African President Nelson Mandela last month. Abacha and his entourage had shown up late for one of the ceremonies.