Backers of Venezuelan President Hugo Chavez prayed and called off New Year's Eve festivities Monday as the cancer-stricken leftist leader took a turn for the worse, fueling doubts about his political future.

Venezuelans prayed in church and a downtown square after the government announced that Chavez suffered "new complications" from a respiratory infection following his fourth cancer-related surgery on December 11 in Cuba.

His vice president and political heir, Nicolas Maduro, broke the news from Havana on Sunday night, saying the condition of the 58-year-old leader was delicate and that he faced an uphill battle.

Maduro decided to stay in Cuba for "the next few hours" to check on the ubiquitous "Comandante," the face of the Latin American left and fierce critic of the United States who has led the oil-rich nation for 14 years.

Jorge Arreaza, Venezuela's science and technology minister as well as the president's son in law, took to Twitter to try to tamp down rampant social media speculation that the end might be near, or had already come.

"My fellow countrymen: do not believe ill-intentioned rumors. President Chavez has spent the day calmly and stable, with his children at his side," said Arreaza who is in Cuba with other family members.

Back in Caracas, crews took down the stage of a downtown concert site while Information Minister Ernesto Villegas invited Venezuelans to gather at Plaza Bolivar to "pray with joy and optimism" for Chavez.

"I deeply love him and would give my life for him. There should be millions like Chavez," Haydee Dominguez, a 50-year-old secretary, said at the gathering led by Villegas.

Others teared up at the San Francisco church while several ministers attended a special mass for Chavez at the Miraflores presidential palace at midday.

At a meeting point for Chavez followers in Plaza Bolivar, "Chavistas" choked up as they contemplated the health of their leader.

"We are all praying for the health of our comandante," said Miriam, one of the people gathered at the square. "There can't be any party here."

Chavez had declared himself cancer free in July, more than a year after being diagnosed with the disease in the pelvic region. The exact nature of the cancer has never been made public.

He was re-elected in October but announced a relapse earlier this month and rushed to Cuba for another operation.

On Monday on Twitter, hashtags translating into expressions such as "Chavez will live and conquer" and "I love Chavez" were numerous, while others speculated about his health.

One of the people discussing Chavez's health was Jose Rafael Marquina, a Venezuelan doctor who lives in the United States and has claimed in the past to have reliable sources informing him about the president.

"The respiratory failure continues without any improvement and the kidney function continues to deteriorate," he wrote on Twitter.

The government has denied such rumors.

Chavez is scheduled to be sworn in on January 10 but the government has indicated that the ceremony could be postponed if the president is not fit by then.

Opposition leader Henrique Capriles, who lost to Chavez in the October 7 election, predicted Monday that there would be "big changes" in 2013.

The opposition coalition, Democratic Unity Table, called for dialogue with the government to deal with what it called an "emergency."

Veppex, a Miami-based association of 25,000 Venezuelans living outside their country as refugees or political exiles, insisted the constitution must be respected verbatim and new elections held if Chavez is indisposed.

Under Venezuela's constitution, a presidential election must be held within 30 days if the head of state is incapacitated or dies before his inauguration or within the first four years of his term.

The government is trying to work out how to "resolve that obstacle," said Luis Vicente Leon, head of pollsters Datanalisis, who said it was clear now that Chavez was in a critical condition.

As the constitution says he must be sworn in on January 10, "anything else will be hard to sell without it being construed as an institutional coup," Leon said.

But Maduro and parliament speaker Diosdado Cabello have left the door open for Chavez to be sworn in at a later date by the Supreme Court.

Cabello has even said that new elections will not be convened on January 10, nor will he himself take over temporarily, as the constitution stipulates, if Chavez is out of the picture.