BISSAU, Guinea-Bissau (AP) — Soldiers have arrested the prime minister of Guinea-Bissau, a military spokesman said Friday, hours after the leader's home was attacked with grenades in what former colonial ruler Portugal described as a military coup.

The attacks that rocked the capital of this tiny country known for cocaine trafficking late Thursday came just two weeks before Prime Minister Carlos Gomes Jr. was to take part in a presidential runoff election as the frontrunner.

Military press attache Francelino Cunha told The Associated Press that Gomes had been detained by the military. The whereabouts of the country's interim president remained unknown.

The Portuguese Foreign Ministry said it "urges the masterminds of the military coup to respect the well-being of the Guinean democratic authorities and free those who have been detained."

However, a communique from an unidentified military commander that was released Friday claims the soldiers don't want to seize power, but instead were trying to halt an invasion from Angolan troops.

Prime Minister Gomes had been favored to win the April 29 runoff after his challenger Kumba Yala, a former president who was overthrown in a 2003 coup, said he would boycott the vote because of irregularities in the first round of balloting.

The special election was being held after Guinea-Bissau's president died in January from diabetes-related complications. Military officials said they thwarted a coup attempt in December not long before his death.

And fears of a military coup have grown since his funeral, when power was handed over to interim President Raimundo Pereira. The chronically unstable nation has been beset by coups since its independence from Portugal in 1974, and its ruler Joao Bernardo "Nino" Vieira was assassinated inside his home in 2009.

Portuguese and American officials warned their citizens against travel to the country.

"The threat of continued violence and an increased potential for political instability and civil or military unrest in Guinea-Bissau remains high," said a statement released by the U.S. Embassy in Dakar, Senegal.

In the communique released Friday in Bissau, an unidentified military commander claimed that Gomes was going to allow troops from Angola, another former Portuguese colony in Africa, to attack military forces in Guinea-Bissau.

Angola sent about 200 troops to Guinea-Bissau in March 2011 to help reform the country's armed forces as part of a bilateral military agreement, according to Angolan state news agency Angop. Their mission recently ended but the contingent is still in Bissau, Angop said without providing further details.

"The Military Command does not want power but it was forced to act in this way to defend itself from the diplomatic maneuvers of the Guinea-Bissau government, which aims to annihilate the (country's) armed forces using foreign military force," the communique said, according to the Portuguese news agency Lusa.

It claimed it possesses a "secret document" drawn up by the Guinea-Bissau government mandating Angola to attack Guinea-Bissau's military. It was impossible to independently verify the claim.

Angolan Defense Minister Candido Pereira Van-Dunem said Thursday in Luanda that his country would "continue to provide full support" to Guinea-Bissau, with which Angola has "excellent ties," Angop reported. He said a calendar for the return of Angolan troops to Luanda was being negotiated with the Bissau authorities.

Explosions rocked the capital, Bissau, Thursday night, according to a diplomat and witnesses. Shooting started after the state radio station signal inexplicably went dead.

Resident Edmond Ajoye, an employee of a Dutch NGO, said he was around 3 miles (5 kilometers) from his home when the shooting began.

"There was panic. Women were running," he said. "There were rockets being launched, and the soldiers were shooting with guns mounted on their trucks."

"The soldiers took downtown," he continued. "The shooting lasted from 7 p.m. until 9 p.m. They then went from embassy to embassy to make sure that the politicians couldn't seek refuge there."

Guinea-Bissau — a nation of 1.5 million people — has weathered successive coups, attempted coups and a civil war. The country has been further destabilized by a growing cocaine trade. Traffickers from Latin America use the nation's archipelago of uninhabited islands to land small, twin-engine planes loaded with drugs, which are then parceled out and carried north for sale in Europe.

The traffickers, according to analysts, have bought off key members of the government and the military, creating a narcostate.

The first round of Guinea-Bissau's presidential election was marred hours after polls closed by the murder of the former chief of intelligence by gunmen near his home.

The unrest in Guinea-Bissau takes place only three weeks after mutinous soldiers overthrew the democratically elected president of Mali, who was about to retire after an April election. The country's junta leader handed over power to an interim civilian president on Thursday.

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Larson reported from Dakar, Senegal. Associated Press writers Lassana Cassama in Bissau, Guinea-Bissau; Michelle Faul and Rukmini Callimachi in Dakar, Senegal; and Barry Hatton in Lisbon, Portugal contributed to this report.