MADRID » The new, left-wing government of Madrid faced its first test over the weekend when the politician charged with running culture was forced to apologize for joking about the Holocaust on Twitter.

On Saturday evening, Guillermo Zapata posted an apology on Twitter, insisting that he was not anti-Semitic. He described the joke as an example of his penchant for "dark and cruel humor," and as a "healthy expression to laugh at the horror that we human beings create." Zapata also noted that the remark was made before he entered politics.

On Sunday, Zapata issued a statement in which he apologized again and strongly condemned anti-Semitism. He added that the joke, posted on Twitter in 2011, had been part of a debate on the limits of humor and was taken out of context. His original post alluded to the incineration of Holocaust victims.

Still, opposition politicians wasted no time in calling for his ouster.

The new mayor, Manuela Carmena, a former judge, came out of retirement to lead a new, local party called Ahora Madrid, or "Now Madrid," in municipal elections last month that underlined the rising fragmentation of Spanish politics. Although Carmena came in second in the voting, Ahora Madrid formed a coalition with the Socialists to remove the Popular Party, which controls the national government, from power, and she was installed Saturday.

Her chief opponent in the election, Esperanza Aguirre of the Popular Party, urged Carmena to demand the immediate resignation of Zapata – if the new mayor did not instead want to become an "accomplice" in Zapata’s anti-Semitic comments.

In the past, Zapata, 35, has also sharply criticized Israel. In 2011, he posted another "cruel humor" post on Twitter about Irene Villa, a journalist who lost her legs in a bombing by ETA, the Basque separatist group.

In a telephone interview later Sunday, he said that after discussing the issue with Carmena, he had no plans to resign. "She didn’t think that what I did was good," Zapata said. "She understands I made a mistake."

"I think one should resign for things linked to the work that one does," he said, "but I haven’t even started my job."

He added that he did not know who dug up his old Twitter posts, but "I understand that, as of yesterday, we have more visibility and there is more preoccupation for what we say or might have said in the past."

Zapata said he had been using "some old jokes in Spain" as examples in the humor debate and he would have apologized in 2011 "if somebody had asked me to, but nobody did."

Carmena, 71, had a distinguished career as a jurist and has steered clear of controversy. Yet the outcry over Zapata underlines the challenge for political new-kids-on-the-block, like Ahora Madrid, which was backed by the left-wing Podemos party and fielded several candidates who had no experience in public life. Zapata, for instance, has been a scriptwriter for television.

Carmena was one of about 8,100 mayors to take office Saturday. In Barcelona, the second-largest city in Spain, Ada Colau, a former housing activist, was elected mayor.

Another Ahora Madrid candidate, Pablo Soto, who is now in charge of transparency for the Madrid government, has posted incendiary comments on social media about conservative politicians. In 2013, for example, he urged that Alberto Ruiz-Gallardsn, a former mayor of Madrid who was at the time the country’s justice minister, be tortured and killed.

Soto, a software developer, did not immediately respond to criticism of earlier attacks against Ruiz-Gallardsn. Spanish law, however, sanctions comments made on social media that could be seen as inciting violence or terrorism.

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