"It was not easy after those seven years of banning political meetings," the director for communications and foreign affairs for the Chadema party, Jon Mrema, told cheering supporters.
Thousands of Chadema supporters gathered at the Furahisha grounds in the lakeside city of Mwanza, draped in the party's blue, red and white colors.
"We have been silent for almost seven years but finally, our right is restored, and we are ready to move ahead," Mwanza resident and party supporter Mary Dismas said.
President Samia Suluhu Hassan this month lifted the ban introduced by her hardline predecessor John Magufuli, who was nicknamed "Bulldozer" for his uncompromising leadership style.
The government's change of heart comes as Hassan, in power for 22 months, seeks to break with some of Magufuli's policies.
The move was cautiously welcomed as a gain for democracy by rights groups and the country's opposition parties.
Magufuli had banned political rallies early in his tenure, saying it was time for work, not politics.
But critics said the ban applied only to opposition groups, with the ruling party free to assemble, and rival gatherings were violently broken up by police and their officials jailed.
Chadema officials said a series of grassroots rallies had been lined up.
"We will organize as many rallies as possible to reach all wards and villages in the country," said Sharifa Suleiman, acting chairperson of the Chadema women's wing.
"This is our time to build the grounds for 2025 elections," she said.
Another official, Hashim Juma Issa, said the party was "opening up a new page" as it celebrated its 30th anniversary.