After a day in which four people were killed and 10 hurt in clashes, dozens of youths took to the streets in western Brazzaville in an apparent bid to throw up barricades as security forces fired rounds of tear gas grenades to hold them back.
Opposition supporters also reported clashes in southern parts of the capital, but it was not clear if today's fresh violence had caused any casualties.
For the second day running, mobile Internet, text messaging services and French radio RFI's signal were cut throughout the city.
The vote proposes changing the constitution to increase the maximum age of presidential candidates, currently 70, and to scrap a rule that limits the number of seven-year terms to two.
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Speaking late yesterday, Interior Minister Raymond Mboulou said the unrest -- which he described as an "organised and coordinated insurrection" -- had left three people dead in Brazzaville and another in Pointe-Noire, the country's economic capital and its second city.
"Symbols of the republic, such as the headquarters of the police (or) the gendarmerie, were targeted," he said.
Pascal Tsaty Mabiala, a leader of the main opposition PanAfrican Union for Social Democracy, called Tuesday for "a peaceful popular insurrection" to prevent the referendum from taking place.
Speaking by phone, Public Works Minister Emile Ouosso who has led a "yes" to the referendum campaign claimed 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.