Ada Colau, who rose to political prominence as part of Spain's "indignados" protest movement, on Sunday was elected mayor of Barcelona, the Wall Street Journal reported. Her leftist coalition was one of several non-traditional parties able to claim triumphs in the country's municipal and regional elections.

"This was the victory of David over Goliath," the 41-year-old mayor-elect said after her win.

Colau's one-year-old Podemos party and another upstart, the center-right Ciudadanos, weakened or broke the grip of Prime Minister Mariano Rajoy's conservative Popular Party (PP) on 10 of the country's 17 regions, as well as on some of its largest cities, the newspaper detailed.

In Madrid, Podemos is best positioned to install its candidate for mayor, meaning the PP might lose its 20-year grip on the Spanish capital's prestigious town hall, according to the Associated Press.

An unnamed senior PP member attending a meeting of the party's executive committee on Monday told Reuters that the election results marked a "time to reflect."

"The party is badly hit. For sure, we're going the wrong way. We are the party that won the most votes but voters sent us a message of anger," the conservative heavyweight said. "We haven't seriously done self-criticism. Something is not working and we have to properly diagnose what," he added.

Rajoy, however, said he planned to keep the focus on the economic recovery and try to better explain his measures to Spaniards; the prime minister, who hopes to secure a second term in office later this year, does not plan to reshuffle his Cabinet or overhaul his strategy, he added.

The economic picture is Spain remains weak, and unemployment is stubbornly high at almost 24 percent. The figure is more than double that for the young, and the government forecasts the overall jobless rate to still hit almost 18 percent in 2017.

Spain's main opposition Socialist Party, which has alternated in power with the PP since the restoration of democracy 40 years ago, meanwhile, viewed Sunday's elections as a vote of no confidence for Rajoy, party leader Pedro Sanchez said.

"It's the beginning of the end of Mariano Rajoy as Prime Minister," he challenged.