Booker made his candidacy official at a news conference in Newark, New Jersey's largest city, which he has led since 2006. He is vying to fill the seat of U.S. Sen. Frank Lautenberg, who died Monday at age 89.
Reps. Frank Pallone and Rush Holt are also planning to enter the Democratic primary. Candidates have until 4 p.m. Monday to file petitions with the secretary of state.
Booker, 44, said he would travel the state to earn every vote. He'll have to hurry; Gov. Chris Christie set the primary for Aug. 13 and a special election for the balance of Lautenberg's term for Oct. 16.
"We need someone in the United States Senate who's actually had to work on difficult problems, who's actually had to find people jobs, who's actually had people standing in front of their homes and had to work on everything from getting people into food stamp programs to helping young people better afford college," Booker said Saturday. A second kickoff event was planned later in the day in the predominantly minority municipality of Willingboro in South Jersey.
Pallone and Holt have yet to announce formal plans. Experts say the race could be an interesting one.
"It will be a question of who can get organizational support from county parties or labor - support from those who will knock on doors and get people out to vote," said political analyst Patrick Murray of Monmouth University. Booker has the national profile and Pallone has more money banked - $3.7 million to Booker's $1.9 million, as of the end of March.
The only Republican running so far is Steve Lonegan, a former Bogota mayor who runs the New Jersey office of Americans for Prosperity.
Booker was joined by former U.S. Sen. Bill Bradley, a former pro basketball player who for 18 years held the Senate seat Booker is seeking.
Bradley called Booker "the right person for the right office at the right time."
Booker started fundraising for a 2014 Senate campaign after announcing he would not run against Christie for governor, citing his desire to finish his term in Newark, which expires in June 2014.
On Saturday, he said he would be sad to leave Newark sooner than planned. But he expressed confidence that $1 billion in development projects in the pipeline in Newark would move forward and that the city's momentum would continue. He said his to-do list before leaving includes continuing to increase transparency and accountability in the city police department and to leave a funding mechanism for hiring police every year.
Booker is considered the early front-runner, though election observers agree anything can happen in a hastily called summer election.
Pallone, 61, has deep union ties. Holt, 64, a former research physicist, has $800,000 on hand.
A Democratic party official in northern New Jersey sued Friday to try to get the special election held Nov. 5, the same date Christie is up for re-election.
The winner of the October special election will hold the seat until November 2014, when voters will elect a senator for the regular six-year term.
Booker pledged not to run a negative campaign against Holt, who he's fund-raised for previously, or Pallone, who has been in Congress since 1993. All three are known to support liberal causes.
Booker has 1.4 million followers on Twitter - or five for every resident of the city where he's the mayor. He tweets frequently, answering questions about city services, posting about his workouts and, perhaps most often, trying to provide inspiration.
He's frequently gotten public attention, from staging a hunger strike to protest drug-dealing to rescuing a woman from a burning home last year. His life story is also captivating. He grew up in Harrington Park as the son of civil rights activists who were among the first black executives at IBM, went to Stanford, was a Rhodes Scholar, earned a law degree from Yale and took a job with the Urban Justice Center, which provides legal and other services to the vulnerable. He also moved to a public housing complex in Newark.
Booker's critics in Newark see him as an ambitious interloper who spends too much of his time outside the city.
In 2010, he was seated next to Facebook founder Mark Zuckerberg at a dinner during a conference in Sun Valley, Idaho. Two months later, Zuckerberg announced a $100 million donation to improve education in Newark. Zuckerberg is planning to host a fundraiser for Booker's Senate run soon.
Associated Press writers Geoff Mulvihill and Katie Zezima in Newark contributed to this story.