A cartoonist who lost his job at the Pittsburgh Post-Gazette believes his searing portrayals of Donald Trump were the most likely cause of his firing.

Rob Rogers was terminated on Thursday by the paper for which he had worked for 25 years, after six cartoons in a row were spiked and his employer tried to change his terms of working, he said.

His last cartoon depicted a bloated man representing the USA, impaled on a steel girder with “trade war” written on it, waving the Stars and Stripes and saying: “Take that, Canada, Mexico and Europe.” After being fired, Rogers drew Trump shaking hands with North Korean leader Kim Jong-un and saying: “You’re so talented and your people love you, look how they’re smiling!”

Kim is standing on a pile of skulls.

“Suppressing voices in any situation is bad,” Rogers told the Guardian. “You want to have as many voices as you can and they are starting to have only one voice of the paper, and I think that goes against what a free press is all about – especially when silencing that voice is because of the president.”

Rogers’s departure prompted uproar from fans including the mayor of Pittsburgh, Bill Peduto. In a statement, he said: “The move today by the leadership of the Pittsburgh Post-Gazette to fire Rob Rogers after he drew a series of cartoons critical of President Trump is disappointing, and sends the wrong message about press freedoms in a time when they are under siege.”

The mayor said he had known Rogers for a long time but that had not stopped the cartoonist criticizing him in his art.

“This is precisely the time,” the mayor added, “when the constitutionally protected free press – including critics like Rob Rogers – should be celebrated and supported, and not fired for doing their jobs. This decision, just one day after the president of the United States said the news media is ‘our country’s biggest enemy’, sets a low standard in the 232-year history of the newspaper.”

Rogers was a finalist for the Pulitzer prize in 1999, for cartoons that skewered then president Bill Clinton, mostly for the Monica Lewinsky scandal. He said he was feeling “anger and outrage” but added: “I saw this coming a while back.”

 After leaving the Post-Gazette, Rogers wrote an editorial for the New York Times headlined: “I was fired for making fun of Trump.”

“When I had lunch with my new boss a few months ago,” he wrote, “he informed me that the paper’s publisher believed the editorial cartoonist was akin to an editorial writer, and that his views should reflect the philosophy of the newspaper. That was a new one to me. I was trained in a tradition in which editorial cartoonists are the live wires of a publication – as one former colleague put it, the ‘constant irritant’.”

Speaking to the Guardian, Rogers said that when he was hired in 1993 the Post-Gazette was a liberal paper, largely reflecting the Democratic leanings of its city. Then as now, John Robinson Block was its publisher. He has been photographed shaking hands with Hillary Clinton and beaming beside Trump on his private jet.

Block did not respond to a request for comment but the Post-Gazette coveredthe firing. Editorial director Keith Burris, whom the paper said “killed” cartoons by Rogers, was quoted as saying Rogers had been offered a new contract and that a “middle way” had been looked for, only for Rogers to be unwilling to “collaborate”.

“We never said he should do no more Trump cartoons or do pro-Trump cartoons,” said Burris, who acknowledged that he is “more conservative” than past editorial page editors. Prior to Trump’s election, Burris said, the owners of the paper had been trying “to right the ship” to reflect less liberal views.

Rogers said “the change in the paper did not happen overnight”, pointing to uproar when the Post-Gazette published an editorial in defence of Trumpwhen he reportedly called a group of developing world countries “shitholes”.

“Things really changed for me in March,” Rogers said, “when management decided that my cartoons about the president were ‘too angry’ and said I was ‘obsessed with Trump’.”

After a series of cartoons satirizing Trump were canceled this month, he said, he was sent a list of new working conditions he called draconian, subjecting him to an unprecedented level of oversight and “clamping down” on his power of free expression. He refused to accept and was fired, he said.

“We are supposed to be the watchdog that keeps the president accountable,” Rogers said, of the press. “In this situation [the Post-Gazette is] coddling the president and giving him cover. I’m worried about the people still working at the paper and about the readers of the largest newspaper in western Pennsylvania.”

Fans of Rogers spoke out on Twitter, reproducing his cartoons. One supporter said: “Dude is talented … we NEED that humor”.

Another posted: “This is the cartoon that got Rogers fired.” It showed a drawing of a road sign saying ‘Caution’ with a silhouette resembling Trump grabbing a child as its parents fled. It was a comment on the controversy surrounding migrant families separated by US officials on crossing the southern border.