Eyewitnesses say Guinea-Bissau soldiers have taken over the streets in the capital, Bissau, in an apparent coup attempt.
Soldiers attacked the home of front runner presidential candidate Carlos Gomes, Jr, the outgoing prime minister.
His whereabouts are unknown.
Witnesses say soldiers also arrested politicians and took control of the ruling party headquarters and national radio station while shots and rocket fire were heard in the capital.
The West African bloc known as ECOWAS announced it condemns any attempted coup d'etat.
The U.S. embassy in Dakar, Senegal, which also covers Guinea Bissau, issued an emergency message to U.S. citizens in the neighboring country to stay away from the downtown area of Bissau. It says the reported violence in the city could be a result of a coup d'etat or a coup attempt, and that an increased potential for political instability and civil or military unrest remains high.
The incidents took place on the eve of the start of the electoral campaign for the runoff presidential election in Guinea Bissau.
Guinea-Bissau's opposition — led by second-place finisher Kumba Yala — has called a boycott of the April 29 runoff vote and warned against campaigning. Yala was one of five candidates that claimed the first-round vote was rigged.
The candidates were vying to replace the late president, Malam Bacai Sanha, who died in January after a long illness.
Since winning independence from Portugal in 1974, Guinea-Bissau has struggled through a dictatorship, three coups and the 2009 assassination of a president.