Government supporters lined the streets, holding placards and banners urging people to vote "yes" in support of Chavez, while the opposition gathered at a separate rally in the capital.
Supporters danced to salsa music booming from sound trucks, before Chavez addressed them.
"Others fight and go to great lengths to divide us, in order to weaken us and dominate us," Chavez said.
"We will turn Venezuela into a world power," he said.
Venezuelans are to vote on changing the constitution to end term limits for mayors and governors as well as for Chavez, whose current term expires in 2012.
Chavez lost a constitutional reform referendum that contained a similar proposal in 2007.