Police have fired shots and tear gas in Congo's capital, Brazzaville, in a clamp down on protests against the president's bid for a third term.
Texting and internet services have been cut and public meetings have been banned ahead of a controversial referendum on Sunday, residents said.
The vote is aimed at pushing through changes to the constitution such as scrapping age and two-term limits.
This would allow President Denis Sassou Nguesso, 71, to run for office again.
Mr Nguesso is one of Africa's longest-serving rulers, first coming to power in 1979 and ruling until 1992 when he lost elections. He returned as president in 1997 after a brief civil war and has since won two elections.
An opposition coalition had planned to hold a rally in the city centre to protest against the proposals to be put to a referendum on Sunday.
Angry youths have burnt tyres in Brazzaville's southern Bacongo and Makelekele districts, sending plumes of smoke into the sky, the AFP news agency reports.
There have been reports that a number of people have been injured in clashes between security forces and protesters.
Other journalists told the BBC that most shops in Brazzaville were shut, and people were staying at home amid fears of violence.
"People are demonstrating across the city. The police are firing tear gas bombs," Tresor Nzila, head of the Congolese Observatory of Human Rights, told the Reuters news agency.
"In certain places, the police have fired warning shots with live fire."
Thousands of protesters waved banners reading "Sassou get out" and "No to the referendum", Reuters reports.
The opposition have been campaigning under the slogan "Sassoufit", a pun on the French expression for "that's enough".
After the government announced a ban on public meetings, saying it expects people to work normally ahead of the referendum, the signal of Radio France International - which is popular in the former French colony - was also cut, AFP reports.
Africa's longest-serving leaders:
- 36 years: Teodoro Obiang Nguema Mbasogo - Equatorial Guinea, took power in a coup in August 1979
- 36 years: Jose Eduardo dos Santos - Angola, took over after death of the country's first president in September 1979
- 35 years: Robert Mugabe - Zimbabwe, won the country's independence elections in April 1980
- 32 years: Paul Biya - Cameroon, took over after resignation of the country's first president in November 1982
- 31 years: Denis Sassou Nguesso - Congo, installed by the military in October 1979, out of power from August 1992-October 1997
- 29 years: Yoweri Museveni - Uganda, became president after his rebel group took power in January 1986