Four people were killed and 10 injured Tuesday in the Republic of Congo, according to officials, as demonstrators braved a government clampdown and took to the streets to protest the president's bid for a third term in office.
The violence prompted urgent calls for calm from a visiting senior US official and Amnesty International.
Clashes erupted after authorities banned a protest rally against next Sunday's referendum on President Denis Sassou Nguesso's bid to extend a three-decade stay in office.
Interior Minister Raymond Mboulou told state TV the "organised and coordinated insurrection" had led to three fatalities in the capital Brazzaville and a fourth in the southern town of Pointe-Noire.
Three members of the security forces were also seriously injured, he added, while 16 arrests were made in Brazzaville.
"The symbols of the republic, such as the police headquarters (or) gendarmerie brigades, were targeted," he said.
The vote proposes increasing the maximum age of presidential candidates, currently 70, and scrapping a rule that limits the maximum number of seven-year terms to two.
"The United States strongly urges all parties, including both the government and the opposition, to engage in dialogue and to refrain from violent actions that would undermine the hard-won peace that all citizens deserve," Sarah Sewall, under-secretary of state for civilian security, democracy and human rights, said at a press conference in Kinshasa.
The human rights group Amnesty also issued a statement, urging security personnel to refrain from using "excessive force" against protesters.
Ahead of the rally, tensions mounted quickly after the president's office broadcast a message on radio and TV saying people were expected to work "as normal" and that gatherings were "banned".
But shops remained shuttered and schools and offices were closed across most of the city as young protesters took to the streets and torched tyres in the southern Makelekele district and in Bacongo in the west.
Police opened fire several times.
Sources at Makelekele hospital said five people were seriously wounded by gunfire, and two others were injured by shrapnel from teargas grenades. An AFP correspondent saw another person being brought to hospital with a gunshot wound to the abdomen.
Makelekele's police station was torched as were two others west of the city centre, witnesses said.
Residents said police fired teargas from helicopters.
Throughout the day in both Brazzaville and Pointe-Noire, the country's economic capital, mobile Internet and SMS messaging services were unavailable as was the signal for France's Radio France Internationale (RFI), one of the country's most popular broadcasters.
Pascal Tsaty Mabiala, a leader of the main opposition PanAfrican Union for Social Democracy, told AFP on Tuesday that the aim was "a peaceful popular insurrection" to try to prevent the referendum from taking place.
He said several people had been shot dead by police on Tuesday, although his claim was denied by government spokesman Thierry Moungalla in remarks to the TV station France 24, who accused the opposition of being "irresponsible".
Speaking by phone, Public Works Minister Emile Ouosso who has led a "yes" to the referendum campaign said he had been "taken hostage" for nearly five hours by "activists opposed to changing the constitution".
Sunday's referendum amendment would allow Sassou Nguesso, who was returned to power in 1997 elections after a brief but bloody civil war, to seek a third term in office.
The former Marxist soldier also ruled Congo from 1979 to 1992.
Under the current constitution, Sassou Nguesso, 72, is not able to run again because of his age and due to the fact that he has already served two seven-year terms.
In the last presidential poll in 2009, he won nearly 79 percent of the votes. Half of his 12 rivals boycotted the election.
Tens of thousands of the president's supporters staged a rally on Saturday in favour of the constitutional changes.
The turnout dwarfed the size of an anti-government demonstration late last month, when several thousand people poured onto the capital's streets to protest against the president's plan to cling to power.
They had rallied under the cry "Sassoufit", a pun on the French expression which means "that's enough".