Hardcore devotees of Brazil’s far-right president, Jair Bolsonaro, have taken to the streets across the country in their first major show of force since his landslide election victory last October.

“Wake up Brazilian people!!! Today is the day to march,” Carla Zambelli, a prominent Bolsonarista congresswoman, tweeted as supporters of the radical populist began gathering on Sunday morning.

Pro-Bolsonaro rallies were reported in more than 300 towns and cities, including Rio de Janeiro, São Paulo, Brasília, and Juiz de Fora, where Bolsonaro was stabbed on the eve of his election, although many of the demonstrations appeared small.

Flag-waving Bolsonaristas also turned out to protest in the Amazon, a region activists fear could suffer irreversible damage under a leader notorious for his hostility to the environment.

“Bolsonaro is not alone, OK? He never has been,” tweeted Éder Mauro, a pro-Bolsonaro congressman from the Amazon city of Belém.

Fernando Sampaio, a Bolsonaro activist who led a demonstration in the south-eastern city of Itabira, said he was marching in support of a leader he believed could rid Brazil of thieving politicians and crime.

“For me, having a president of the republic who isn’t corrupt is already a massive victory for our country,” Sampaio said.

Bolsonaro did not take part in the controversial mobilization, which critics have slammed as a dangerous attempt to radicalise supporters and bully Brazil’s democratic institutions into backing the president’s plans.

But he tweeted his approval, sharing cellphone videos of the rallies with his 4.3m followers.

“Today is the day that the people are taking to the streets to defend the future of this nation,” Bolsonaro told reporters after attending a church service in Rio, where protesters marched down Copacabana beach wearing Trumpian caps reading “Make Brazil Great Again”.

Bolsonaro supporters in front of the national congress in Brasília on 26 May. Photograph: Evaristo Sa/AFP/Getty Images

Monica de Bolle, director of Latin American studies at Johns Hopkins University, said Bolsonaro hoped to use the rallies to gauge his support amid growing criticism from some disillusioned portions of the Brazilian right.

“It’s a barometer more than anything else … He’s trying to assess what kind of support he has,” De Bolle said.

Despite Sunday’s show of strength from loyalists, opinion polls suggest the answer is less and less.

Five months into Bolsonaro’s four-year term his approval ratings have plummeted with Brazil’s economy stuttering, political infighting raging and the president facing uncomfortable questions over his family’s ties to organized crime and a corruption scandal involving one of his sons.

Bolsonaro has also faced international repudiation as a result of his extremist views, recently cancelling a visit to New York because of protests against him there.

In an editorial on Sunday the conservative Estado de São Paulo newspaper said support for Bolsonaro “was melting away before our eyes”, with about 36% of voters now considering his administration bad or awful, compared to just 17% in February.

The majority of Brazilians who were not seduced by Bolsonaro’s “salvationist gobbledygook” had little left but pessimism, the newspaper added.

Waning support for Bolsonaro has sparked growing chatter about his possible impeachment among political observers.

“I think the paths for him finishing out his term are getting very, very narrow,” said De Bolle.

“We have never had a president without political support [and] without popular support who has been able to make it to a full four-year term – that’s just never happened.”

But Sampaio insisted Bolsonaro – a former army captain who has vowed to stamp out crime and corruption – was the man for the job.

“Brazil’s biggest problem … is the 70,000 or so homicides we have each year … Brazil is in a world war,” he said. “I trust in the president’s work.”