LAGOS, Nigeria -- Police sealed off the headquarters of Nigeria's three main labor groups Thursday in another attempt by the military to take over pro-democracy unions that have paralyzed the nation with strikes.


Squads of police also were stationed at major intersections and bus terminals around Lagos, the center of the anti-military movement.


But violence broke out elsewhere. About 5,000 people chanting anti-government slogans rioted in Benin City, 240 kilometers east of Lagos, and set ablaze a hotel owned by the labor minister, a former general.


On Tuesday, protesters firebombed two mansions owned by the minister.


The country's military ruler, General Sani Abacha, announced Wednesday that he will replace the leaders of the two oil unions that initiated the strike and the 5 million-member Nigeria Labor Congress with his own administrators. But labor leaders said they were ignoring the declaration.


"We will fight to the last. We must halt this arbitrary dictatorship," said Frank Kokori, general secretary of the National Union of Petroleum and Natural Gas workers, which has threatened violence in response to military oppression.


Kokori said oil union leaders would meet secretly Thursday to plan a united response.


The country has been in crisis since the military annulled a June 1993 presidential election apparently won by the businessman Moshood Abiola. The election was supposed to end a decade of military rule. Abacha toppled a civilian figurehead in November, dissolved all elected state and national legislatures, closed newspapers and jailed critics.


Abiola was arrested after declaring himself president to mark the first anniversary of the election. Oil workers went on strike in protest on July 4, crippling the industry that provides 80 percent of government income.


In his nationwide broadcast, Abacha said he would not free Abiola, on trial for treason, claiming that decision would be up to military-appointed judges.


"It is a selfish motive, ruthless in execution and sadistic in expectations," Abacha said in the broadcast.


Abacha has been unable to contain the effects of the strike. Nigeria's oil exports have been cut in half, and there have been growing signs of an organized campaign of violence against the dictatorship.


Nigeria, among Africa's richest nations, has been plagued by oppressive, incompetent and corrupt military dictatorships since independence in 1960.