The Department of Homeland Security said it detained Luis Posada Carriles on Tuesday, after the longtime Castro opponent granted interviews to TV stations and The Miami Herald for the first time since surfacing in the United States two months ago. Posada, a former CIA operative and Venezuelan security official, is wanted in the 1976 bombing of the Cuban airliner that killed 73 people.

Cuba's Parliament speaker welcomed the news of Posada's detention, but questioned why the U.S. government took so long.

''Do you want us to applaud the fact that he has been arrested after his presence [in the United States] was burning for two months?'' Ricardo Alarcon asked in an interview with The Associated Press.

The Cuban leader's campaign for Posada's arrest has been the most intense media battle he has waged since the international custody struggle over young Cuban castaway Elian Gonzalez.

The size of the crowd in the ''March against Terrorism'' was similar to those organized in 2000 to press for Elian's return to his father.

''This is not a march against the people of the United States,'' said the 78-year-old Castro, differentiating between Americans and their government. ''It is a march against terrorism, in favor of life and of peace.''

''Our country has been the object of the most ferocious war in history,'' Castro added in brief comments to the crowd, referring to U.S. economic sanctions against the island and early U.S. government plots to kill him.

Wearing his traditional olive green military uniform and cap, the Cuban president then walked six or seven blocks past the American mission without assistance, despite an accidental fall in October that shattered his left kneecap.

Alarcon said he didn't know if the pressure from Cuba had contributed to Posada's arrest, ''but if we have made a contribution toward justice being done, I am happy.''

He said he thought much of the pressure came from within the United States, a nation especially sensitive about terrorist acts following the 2001 attacks on New York and Washington.

''I have the impression that the immense majority of Americans believe that terrorism is a grave matter,'' Alarcon said.

For more than four hours, huge rivers of people, many chanting and waving tiny Cuban flags, spilled from side streets onto the Malecon coastal highway running past the mission. ''Punishment for the assassins!'' a child shouted over a loudspeaker. ''Bush, terrorist!'' chanted another.

Posada, 77, arrived in the United States in March and soon after requested political asylum. Venezuela, which has an extradition treaty with the United States, last week formally requested his extradition in the 1976 bombing. Posada has denied any role in the bombing.

The Homeland Security department did not say what it planned to do with Posada after detaining him. But it said that generally, the U.S. government does not return people to Cuba or to countries acting on Cuba's behalf. The department said it has 48 hours to decide his status.

''Now Mr. Bush has to prove he is sincere about terrorism,'' Alarcon said in the Cuban government's first official reaction to the arrest of the man Fidel Castro has called ''the most famous and cruel terrorist of the Western Hemisphere.''