The head of the outgoing government faces three challengers who were unknown when he first took power in 2011: rising star Albert Rivera, 36, from the centrist Ciudadanos party, the Socialists' Pedro Sanchez, 43, and far-left Podemos candidate Pablo Iglesias, 37.
"We have 800,000 members," Rajoy told supporters of his Popular Party (PP) in Madrid as he highlighted his experience and repeated the slogan, "We take Spain seriously."
"We are going to win these elections," the 60-year-old added, though analysts have predicted this could be the tightest nationwide vote the country has seen in decades.
The PP is extending its lead over rivals, according to a poll published Thursday, with a projected 28.6 percent of the vote, down from the 44.6 percent it won during the last election.
But the same poll found that 41 percent of voters were still undecided.
The Socialists' Sanchez also kicked off his campaign Thursday with a high-octane rally in a hall near Madrid, where he played messages of support from a number of European leaders, including Italy's Matteo Renzi.
Rivera officially launched his bid in a Madrid hotel, with posters featuring his name and the message, "with hope".
The election is likely to expose fault lines between Spain's rural, older population -- who largely favour the traditional parties -- and younger urbanites who lean towards Podemos and Ciudadanos.
The Popular Party has been worn down by corruption scandals, austerity measures and an unemployment rate that is falling but still stands at 21.1 percent, the highest in the eurozone with the exception of Greece.