At least 20 people have died in Ivory Coast in clashes between troops and demonstrators, says a spokeswoman for incumbent president Laurent Gbagbo.
Ten protesters and 10 members of the security forces had died, she said.
Troops loyal to Mr Gbagbo have clashed with supporters of Alassane Ouattara, who has been staying at a hotel in Abidjan since the disputed polls.
Both men claim to have won the election, and each has declared himself president.
The UN Security Council has expressed deep concern over the violence.
It warned that all sides would be held accountable under international law for any attacks against civilians.
Most of the violence was reported in Abidjan, but on Thursday afternoon it appeared to be spreading, with reports emerging of unrest elsewhere.
Former rebels from the New Forces fighters apparently tried to storm positions held by the military near the town of Tiebissou, just south of the ceasefire line agreed in 2003 to end the country's year-long civil war.
The sides exchanged fire and civilians fled the town, according to reports, but there were no confirmed casualties.
Meanwhile in Abidjan, soldiers and police were standing guard at almost every major road junction.
Violence broke out in several parts of the city as security forces tried to prevent crowds from gathering.
Mr Ouattara's supporters had planned to take over state media channel Radio Television Ivorienne (RTI), which has been broadcasting reports praising Mr Gbagbo since the disputed election.
But early on Thursday, they clashed with security forces, who opened fire on the crowds.
Separately, in the south of the city, gun battles erupted between soldiers who control the area and former rebels who are guarding the Golf Hotel, where Mr Ouattara has been based since the poll.
"There is shooting all over the place, there is artillery, there are explosions," a witness told Reuters news agency.
AFP news agency quoted a UN source as saying that 800 peacekeepers had been sent with supplies to the hotel.
Mr Ouattara's supporters, including his nominated prime minister Guillaume Soro, were trying to leave the hotel compound and march to the TV station when fighting broke out.
Kehi Edward, a spokesman for Mr Ouattara, told the BBC that they were being blockaded.
"The military service, the factions close to Mr Gbagbo, have closed both ends of the hotel so there is no traffic in and no traffic out of the hotel," he said.
US officials said the US embassy, which is near the Golf Hotel, had been hit by an "errant rocket-propelled grenade" during the battle, but no-one was injured.
Earlier in the day, international prosecutors said they would bring legal proceedings against anyone accused of causing deaths.
And UN Secretary General Ban Ki-moon's spokesman warned that "those who incite or perpetrate violence, and those who use the media for that purpose... will be held accountable for their actions".
He reiterated that the UN, which has about 10,000 peacekeepers in the country and is helping to guard the Golf Hotel, wanted Mr Gbagbo to stand down.
The trouble stems from last month's run-off election, which the Electoral Commission said Mr Ouattara won by 54% to 46%.
Mr Gbagbo refused to admit defeat, and the Constitutional Council then annulled some results from the north and declared Mr Gbagbo the winner.