Abidjan, Ivory Coast (CNN) -- At least nine unarmed protesters in Ivory Coast's largest city were shot and killed by security forces Thursday, eyewitnesses told Amnesty International.
The violence erupted as troops loyal to the incumbent president and supporters of his challenger confronted each other on the tense and chaotic streets of Abidjan.
"Amnesty International is appalled by this completely unjustified and disproportionate use of force and calls on the Ivorian security forces to stop these killings immediately," said Salvatore Sagues, the West Africa researcher for Amnesty, a human rights group.
"Those who opened fire on these people, as well as those who gave the order, will have to account for their acts," he said.
The demonstrators -- part of mass street protests called by the presidential challenger, Alassane Ouattara -- had been marching from various locations in Abidjan "in an attempt to seize the state-run RTI" broadcasting offices when security forces fired on them, Amnesty said.
Witnesses told CNN that exchanges of fire also raged outside a hotel that is serving as the headquarters of Ouattara, a former economist and prime minister who ran for president against incumbent President Laurent Gbagbo in a November 28 runoff.
The West African nation has been in a political stalemate over the disputed election, in which both men claim victory.
The country's Independent Electoral Commission said Ouattara won the runoff, but the country's Constitutional Council invalidated those results and declared Gbagbo the winner.
Ever since, tension has filled the air in Abidjan. On Tuesday, Amnesty International said, "Ouattara called for mass street protests to seize state radio and government buildings, still held by officials loyal to Laurent Gbagbo, who is refusing to leave office."
Eyewitnesses told Amnesty International at least six people were shot dead by security forces in the neighborhood of Abobo.
"When we saw security forces near the police station of the 21st arrondissement, we raised our hands and told them that we were unarmed. They refused to answer and threw tear gas grenades. Then immediately they shot at us. I saw dead bodies in the street. I know three of them," one eyewitness told Amnesty International.
Security forces near the Makassi crossroads shot tear gas and then fired on another group of protesters, killing three people, the human rights organization said.
"A young man, a car washer, who was not participating in the demonstration, was shot dead by two men wearing military uniforms, with hoods. The young man was standing in front of the garage waiting for drivers in order to wash their vehicles when two military (personnel) arrive. One of them tripped him up, he fell down. One of the military who was wearing sunglasses shot at him at point blank with a gun," an eyewitness told Amnesty International.
CNN witnesses saw at least two dead bodies.
One of the bodies was seen in front of U.N. headquarters, where there was a tent for the wounded. Another was seen in a slum near the Hotel Golf. Witnesses said he was a pro-Ouattara supporter who was shot by police because he was armed, but that account could not be confirmed.
Heavy gunfire and mortars could be heard for at least an hour and up to three miles away from the hotel, where people protecting the Ouattara headquarters exchanged fire with forces loyal to Gbagbo.
Soldiers loyal to Gbagbo were in combat posture, hiding in the bush around the hotel. Ex-rebel forces were trying to free blockades held by the pro-Gbagbo forces on the sole access road to the hotel.
Witnesses spoke of confrontation at the RTI building, which has been surrounded by armored vehicles and troops loyal to Gbagbo since the crisis began.
Also, police forces fired shots to disperse demonstrators in a pro-Ouattara neighborhood, witnesses said.
The United Nations, African Union, European Union and other international bodies and leaders have called on Gbagbo to step down, but he has declined to do so.
On Wednesday, U.N. Secretary-General Ban Ki-moon warned that the stalemate could lead to renewed fighting in a country that was wracked by civil war for several years.
"The situation is taking a worrying turn with unfolding events that could lead to widespread violence," Ban said in a written statement.
The developments include recent threats by some commanders of the national security forces to resort to military options and also clashes on Monday between them and ex-rebels supporting Ouattara, the internationally recognized winner of the election.
The International Criminal Court's chief prosecutor, Luis Moreno Ocampo, told French media that he would prosecute people responsible for any deadly post-election violence.
Sagues also warned that Ivory Coast is close to another civil war.
"Every effort must be made to prevent an escalation of violence. There is a very real threat that many more lives will be lost if the security forces continue to shoot at protesters indiscriminately," Sagues said.
Journalist Eric Agnero contributed to this report