Witnesses said the coup soldiers exchanged sustained fire with Nigerian troops around the presidential office complex in the West African nation's capital, Freetown.
The coup leaders imposed a night curfew and warned looters, military or civilian, they would be shot on sight. They ordered all military personnel, civilian ministers and politicians to report to army headquarters.
In Washington, the United States urged its citizens in Freetown to stay indoors and said it was prepared to evacuate them after its embassy there was hit. No one was hurt.
"An exchange of fire between Nigerian troops and coup-makers continues at State House," said one person who lives near the city centre complex. Gun battles raged most of the day.
There was no immediate word on casualties from the clash. Hospital sources said five civilians died elsewhere in the capital and several civilians and soldiers were wounded.
Nigerian and Guinean troops have backed the army in Sierra Leone's rebel war. Nigerian troops guard the State House, the capital's international airport and other key sites.
Coup spokesman Corporal Gborie said in a radio broadcast that junior ranks had toppled Mr Kabbah, who had fled to neighbouring Guinea. He asked Nigerian troops not to intervene in what he called an internal matter. One official said Nigerian troops had tried to prevent the Sierra Leonean soldiers entering certain areas.
"We are informing our counterparts, the Nigerian forces, to know that this is a purely internal matter," the coup spokesman said on the radio. "We want them to put down their arms and return to their various deployment areas immediately."
Small-arms fire and the occasional crash of mortar shells continued into the afternoon. A gov- ernment building in the vicinity of the State House was on fire and other office blocks, including the treasury, were threatened, residents said.
At the start of the coup, about 20 heavily armed men stormed Freetown's maximum security prison and freed some 600 inmates, witnesses said. They included some soldiers charged with plotting against Mr Kabbah.
A White House spokesman, Barry Toiv, said the US embassy was hit twice by rocket-propelled grenades. No one was hurt.
"We have advised US citizens in Sierra Leone to stay indoors and we've made preparations, in the event that there needs to be an evacuation, but there is no evacuation going on at this point," Mr Toiv said.
In the city centre, the headquarters of UN agencies was ransacked and vehicles parked there removed, UN sources said. Shops and businesses closed yesterday were pillaged, mainly by men in uniform, but also by some civilians, witnesses said.
The coup-makers issued no orders regarding the country's sea and airports or land borders, but the situation at these points was not immediately clear. A sergeant said the army had seized parliament and government offices at State House, as well as the radio and television. He said a new government would be announced by the end of the day.
The soldiers called for the return to Sierra Leone of Foday Sankoh, leader of the rebel Revolutionary United Front (RUF), who has been kept in a hotel suite in the Nigerian capital for the past two months by Nigeria's military authorities.