The rally came ahead of an October 25 referendum that will determine whether the veteran leader -- who has already had three decades in power -- can seek another presidential term.
The constitution currently bars the 72-year-old from running again as there is an age limit of 70 as well as a ceiling of two mandates.
The Congolese government said Monday that the referendum would be on scrapping both the two-term limit and the age cap.
Tens of thousands thronged the city centre on Saturday, AFP journalists saw, with some Sassou Nguesso supporters at the event wearing T-shirts reading "Yes to changing the constitution" and "Let's go vote".
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Banners floated above the crowd reading "Yes to the Referendum" and "Sassoui" -- a play on the leader's name and the French word for yes.
"Our vote will be yes on October 25," Land Affairs Minister Pierre Mabiala, a member of Sassou Nguesso's PCT party, told the crowd.
Today's turnout dwarfed that of an anti-government demo 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 for "that is enough".
"They have not achieved what we achieved, despite the fact that they distributed money and gadgets. They even intimidated civil servants into attending the rally," opposition leader and president of Congo's Social Democrats (PSDC) told AFP.
"There weren't any real supporters of the 'yes' vote... We are convinced that the majority will refuse," he added.
The upcoming referendum has attracted international criticism, with Washington conveying its "concern" over Sassou Nguesso's decision to try to change the constitution to stay in office.