The poll is seen a test of the country's recovery after an 11-year civil war that killed more than 50,000 people.
"I hope the people of Sierra Leone can have this election without violence. All that is needed is one act of violence to spark another civil war."
Tom Dougherty, Atlanta, USA
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A Reuters news agency reporter witnessed around two dozen wounded people being taken to a hospital in a police van, while Tamba Gbekie, a police commander, said at least three people had been stabbed.
Witnesses on the city's streets reported hearing gunshots but police could not confirm whether they came from the crowd.
"They are throwing stones at us and firing," Joseph Gbla, an unemployed 26-year-old SLPP supporter, said.
"They don't want the peace to go further. They want to create chaos."
Koroma, of the All People's Congress (APC), won the first round of voting on August 11 with 44 per cent of the vote, amid widespread discontent at widespread corruption and high unemployment.
Soloman Berewa, the vice president of the SLPP, came second with 38 per cent.
He now faces the difficult task of winning the September 8 run-off after Charles Margai, the candidate who came in third, threw his support behind Koroma. Ethnic divisions
Police set up roadblocks along Freetown's main street, separating the two groups of supporters as groups of young men chanting SLPP slogans marched through the streets.
The disturbances later spread to the east of the city, an APC stronghold, where gangs of young people, dressed in the party's red colours, tore down Berewa posters.
Police again used tear gas to break up the crowds.
"If you wear red they attack you," Samuel Koroma, 23 and unemployed, said. "The ruling government wants war because we don't want them any more."
The elections have revealed ethnic fault lines in the nation of more than five million people, with the SLPP drawing its support from the southern Mende peoples and the APC proving stronger in the north and west Temne region.
Meanwhile, at a meeting on Saturday, Berewa and Koroma agreed to a peace march on Monday in an effort to defuse political tensions.
But observers say their efforts are meaningless unless the parties controlled their sympathisers.
"The skirmishes in Freetown really underscore the need for the leadership to call their supporters to order and to have civil conduct and peaceful elections," Chris Fomunyoh, of the National Democratic Institute's (NDI) observer mission, said.