People walk near smoke rising from burning tyres at a street in Abidjan Dec 16, 2010. Heavy weapons were fired near the Abidjan base of the internationally-endorsed winner of a disputed presidential poll on Thursday before a planned march by his supporters to seize the state broadcaster, witnesses said. [Photo/Agencies]

ABIDJAN, Cote d'Ivoire (AP)- Forces loyal to the two men claiming Cote d'Ivoire's presidency clashed in the streets of the commercial capital Thursday, killing at least 20 people and bolstering fears that the world's top cocoa producer is on the verge of another civil war.

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Explosions and gunfire were heard throughout Abidjan- once known as the "Paris of Africa" for its cosmopolitan nightlife and chic boutiques.

An errant rocket-propelled grenade struck an outer perimeter wall of the U.S. Embassy, but no injuries were reported and the damage was minor, State Department spokesman P.J. Crowley said in Washington.

Cote d'Ivoire has been operating with two presidents and two governments since a disputed November 28 runoff election. Alassane Ouattara was declared the winner by the country's electoral commission and was recognized by the U.N., U.S., France and the African Union as having beaten incumbent Laurent Gbagbo. The next day, however, the constitutional council overturned the results after invalidating a half-million votes from Ouattara strongholds.

Crowley said a combined delegation from the African Union and the Economic Community Of West African States would be in Abidjan soon "to continue to encourage President Gbagbo to step aside."

The bloodshed in Abidjan is part of a risky push by Ouattara to take control of state institutions after the balloting that many hoped would reunite the West African nation following a 2002-03 war that split it in two.

Amnesty International warned that the regional powerhouse "has never been so close to a resumption of civil war."

"Every effort must be done to prevent further escalation of violence," the group said.

The U.S. and other nations have given Gbagbo an ultimatum to step down and leave Cote d'Ivoire within days or face travel and financial sanctions, a senior Obama administration official said.

The official said the U.S., the U.N, France and the African Union had passed the message to Gbagbo's camp that "he has to go," and that there were signs he might agree. The official spoke on condition of anonymity to describe the ongoing discussions.

The official would not describe the signs, but noted that Gbagbo and his family have "multiple homes in multiple countries" that they would not be able to use if sanctions are imposed. The U.S. and the European Union have all threatened sanctions on Gbagbo, his family and his supporters if he does not accept the election result.

Streams of gunfire and unexplained explosions were heard for 30-45 minutes in the streets outside the U.N.-protected Golf Hotel, where Ouattara has tried to govern. Gbagbo rules from the presidential palace.

The shooting erupted when rebel troops who control the north of the country and are helping guard Ouattara tried to remove makeshift roadblocks near the hotel, Ouattara communications adviser Massere Toure told The Associated Press.

The deputy spokesman for the U.N., Farhan Haq, said mortars and other heavy weapons were used in the fighting.

He said the U.N. has deployed nearly 800 military and police and eight armored personnel carriers at the Golf Hotel to provide security together with French troops and rebel forces. He said the U.N. had stocked up on water, generators and fuel at the hotel.

Casualty tolls for the day's violence varied.

Gbagbo's minister of education, Jacqueline Oble, confirmed 20 deaths in a statement read on state television, but specified that 10 of those were police killed by protesters who she said fired on the officers.

Senior opposition official Amadou Coulibaly put the toll at 30 dead.

Traore Drissa, a lawyer who runs the Abidjan-based Ivorian Movement for Human Rights, also said 20 people were killed, but did not give a breakdown of whether they were protesters or police. Amnesty International counted at least nine bodies.

The army and police _ both loyal to Gbagbo _ declined to comment on the fighting.

U.N. Secretary-General Ban Ki-moon has warned that the politically charged environment could spark a new civil war.

The United Nations called all parties to show restraint and to remain calm. The secretary-general's special representative, Choi Young-jin, has been in contact with both sides to try to calm the situation.

Riot police fired tear gas and live rounds to disperse protesters in multiple parts of the city. In the Abobo neighborhood, an AP photographer saw the bodies of three men who several witnesses said had been shot by police. One had been shot in the head, the other two in the chest. Several more were wounded in midmorning clashes elsewhere, according to AP reporters on the scene.

A small group of Ouattara supporters marching in the Koumassi marketplace early in the day were confronted by police who pulled one young man from the crowd.

"They shot him in the stomach with a tear gas canister at point-blank range," said protester Ahmed Konate. "He fell to the ground in a cloud of gas and didn't get up. He was dead."

The violence brought skyscraper-lined Abidjan to a standstill. Businesses were closed and fearful residents stayed home. Streets were deserted except for soldiers and police, who also used batons to beat back demonstrators, some of whom hurled stones from rooftops at security forces.

The dispute has raised fears of renewed unrest in a country known for decades as a beacon of prosperity and stability in a part of Africa better known for coups and war; its cocoa plantations attracted millions of immigrants from neighboring nations.

Ouattara draws much of his support from the rebel-held north, while Gbagbo's power base is in the south.

Amnesty International's West Africa researcher Salvatore Sagues said it was "appalled by this completely unjustified and disproportionate use of force."

"Those who opened fire on these people, as well as those who gave the order will have to account for their acts," he said.

Drissa said clashes had also broken out in the capital of Yamoussoukro as well as the rebel stronghold of Bouake and the central town of Tiebissou.

Ouattara plans a second march Friday to take back other government buildings and hold a Cabinet meeting.

"The next two days will determine everything. It's all or nothing," said Jean-Claude N'dri, a cable television salesman in Treichville neighborhood, where riot police and soldiers loyal to Gbagbo fired tear gas to disperse about 500 people.

Similar violence broke out in the city's Cocody district. And outside the opposition coalition headquarters, police in armored vehicles fired into a crowd of hundreds of demonstrators, wounding three people, said Michel Bazia, a civil servant who lives in the neighborhood.

Ouattara has called on his backers to help him take control of state institutions. On Thursday, they had vowed to march to the national television station to install a new state TV chief but did not get close to the building.

It is heavily protected by Gbagbo's troops, and police and soldiers sealed off streets around it with roadblocks and parked APCs.

The two stations in the building are the only Ivorian broadcasters in the country. They are a powerful voice for whoever controls them: In the days after the U.N. said Gbagbo lost, people watching Gbagbo-controlled state TV saw only the announcement of his victory.