The “march for change” was the most important rally organized by Podemos since it emerged as a newcomer in Spanish politics last May, when it won almost 8 percent of the Spanish vote in elections for the European Parliament.
That result — just a few months after the party’s official formation — helped deny the governing conservative Popular Party and the opposition Socialists a majority of votes for the first time since the return to democracy in the late 1970s.
Since then, Podemos has made further gains in opinion polls, raising the prospect of a three-party race in an election year that starts in March with a regional vote in Andalusia, Spain’s largest region, and culminates in general elections in the fall.
The march on Saturday ended in the Puerta del Sol, the Madrid square that became the center of a nationwide, youth-led movement seeking to overhaul Spain’s political system in May 2011.
The movement, however, then lost steam. Six months after protesters occupied the square and urged voters to reject traditional parties, the Popular Party of Prime Minister Mariano Rajoy swept to power in late 2011.
But this year, “things will be very different because we’re now much more organized and we have Podemos, a party that is going to go from strength to strength as each election takes place,” said José Vicente Moreno, who traveled to Madrid with about 300 people from the province of Castellón, in eastern Spain, to join the march.
In Greece, Syriza, another left-wing party, has combined an anti-austerity message with a pledge to renegotiate the terms of the country’s debt. Ahead of the Greek vote last Sunday, Pablo Iglesias, the leader of Podemos, joined Alexis Tsipras, the leader of Syriza, at a political rally in Athens.
Addressing the Madrid crowd on Saturday, Mr. Iglesias promised that Podemos (which means “We Can”), would soon oust the Popular Party from power. He accused the government of Mr. Rajoy of “wanting to humiliate our country with this scam that they call austerity.”
Rubén Aguilar, a Spanish telecom technician, was waving a Greek flag as he marched on Saturday, but “out of solidarity and not because I think Spain is Greece.”
“We’re better off economically than our Greek friends,” he added, “but we share their determination to put the interests of people back ahead of economic goals like debt repayment.”
After six years of crisis, Mr. Rajoy’s Popular Party is now hoping to persuade voters that the country’s economic turnaround is meaningful. Unemployment has fallen slightly in recent months, as Spain’s economy grew 1.4 percent last year, according to data released in the past week.
But Mr. Rajoy’s party is also entangled in a major corruption scandal that centers on whether its former treasurer ran a party slush fund. Spain’s other traditional parties are also involved in fraud court cases of their own.
Angeles Buj, 61, said Saturday that the economic recovery was “perhaps benefiting further our corrupt political leaders but doing zero for those who’ve really suffered in this crisis.”
Ms. Buj, who held a sign that read “Yes, united, we can,” said that she was “among the lucky ones who have work,” but that she would vote for Podemos because “it’s time to give some dignity back to the Spanish people.”