Soldiers in Guinea-Bissau have launched a coup attempt, taking over roads, broadcasting facilities, and government offices in the country's capital.

The takeover attempt began late Thursday, the night before campaigning was due to begin for a presidential run-off election.

Shots and explosions were heard in the capital, Bissau, and soldiers attacked the home of front-runner presidential candidate Carlos Gomes, Jr., who is said to be unpopular with the military.

The whereabouts of Mr. Gomes and interim President Raimundo Pereira were unknown early Friday.

A statement attributed to Guinea-Bissau's “Military Command” said the army does not want to take power. It said soldiers were forced to act because of an alleged secret agreement between Guinea-Bissau and Angola that would allow Angolan forces to attack Guinea-Bissau's military.

Witnesses say the capital was generally calm Friday, though local radio stations were off the air.

The West African bloc ECOWAS said it condemns any coup attempt. The U.S. embassy in Senegal, which also covers Guinea-Bissau, urged the military to restore civilian leadership.

The embassy warned Americans in Guinea-Bissau to avoid the downtown area of the capital.

Guinea-Bissau's opposition — led by second-place finisher Kumba Yala — had called a boycott of the April 29 presidential runoff 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. The country is also a conduit for traffickers shipping drugs to Europe.