NEWARK, N.J. (AP) _ Three men accused of trying to buy $200,000 worth of weapons were planning a coup in their Ghanian homeland, a defense attorney says, calling the African nation's ruler a ''usurper.''

''It's not a spy case of a high-tech case,'' attorney J. Jeffrey Weisenfeld said at the arraignment of the three Monday. ''It's not a crime of violence or even of moral turpitude.''

But U.S. Attorney Thomas Greelish successfully argued that the men should remain in custody. ''Procuring fragmentation grenades and grenade launchers threatens the bodily harm of somebody, perhaps not here,'' said U.S. Magistrate Serena Perretti in ordering they be held without bail.

Weisenfeld represents Kwasi John Baidoo, 40, of Rockaway Township, who was arrested Sunday at a parking lot near Newark International Airport along with Joseph Henry Mensah, 57, of London, England, and John Andrews Boateng of Brooklyn, N.Y. They were charged with conspiracy and violating the Arms Export Control Act.

Greelish told Perretti the three were trying to arm ''a 100-man army for the overthrow'' of the Ghanaian government.

Ghana is ruled by Jerry John Rawlings, an air force officer who seized power in 1982. Weisenfeld called Rawlings ''a usurper who has taken over the government of Ghana.''

The defendants face five-year prison terms if convicted of conspiracy, but Greelish said he expects more charges will be filed after a federal grand jury reviews the case.

Mensah was finance minister in the civilian government of K.H. Busiah from 1969 until it was toppled by the military in 1972. He was jailed twice before being released in June 1978.

Mensah then moved to London, and in March 1984 announced formation of the Ghana Democratic Movement, inviting participation of all Ghanaians ''who believe in the restoration of democracy in Ghana and are prepared to fight for it.''

Greelish said the three men, in addition to agreeing to pay $200,000 for weapons, also were willing to pay $50,000 to bribe Ghanaian officials to overlook the illegal import into their nation.

The shopping list also included antitank weapons, machine guns, mortars, antiaircraft weapons and grenades, authorities said.

U.S. Customs Agent Art Stiffel said Boateng is a cab driver in Brooklyn, Baidoo a computer technician for Nabisco Brands in Hanover Township and Mensah an economic consultant. Stiffel said Boateng and Baidoo, members of the Ghana Democratic Movement, are resident aliens in the United States. They face deportation if convicted.