President Luis Cabral of Guinea-Bissau was overthrown in a nearly bloodless coup staged by black-power African nationalists in the Army, contributor Richard Timsar reports. The country's new leader is Prime Minister Bernardo Joao Vieira , a former guerrilla leader in the war against the Portuguese, who received military training in China and Cuba. The coup led to an immediate break with Cape Verde, a group of islands 400 miles off the west African coast. Until Friday, both countries had been ruled by the African Party for Independence in Guinea-Bissau and Cape Verde (PAIGC), and the party proclaimed the unification of the two countries to be its ultimate goal. But initial broadcasts by the new leadership in Bissau made clear that black Guineans had taken power in their own hands, and troops started rounding up PAIGC and government leaders of Cape Verdean origin.