A Tanzanian investigative journalist has appeared in court charged with organised crime and money laundering.

Erick Kabendera, who was arrested by plainclothes policemen last week, appeared in court charged with leading organised crime, failure to pay tax amounting to 173m Tanzanian shillings ($75,000) and money laundering of the same amount. Press freedom advocates have called the charges “clearly retaliatory”.

The charge sheet said he “knowingly furnished assistance in the conduct of affairs of a criminal racket, with intent either to reap profit or other benefit”.

Kabendera, whose work has appeared in the Guardian and many other publications, has recently published stories about political infighting and an alleged plot to stop the re-election of the Tanzanian president, John Magufuli, who has been criticised for a sharp increase in repression since he took power in 2015.

Kabendera seemed calm as he heard the charges in Kisutu resident magistrates court in the former Tanzanian capital, Dar es Salaam, after being held for a week by police with very limited access to his family and lawyers. This is against Tanzania’s constitution: a person has to be charged in court after being detained for 24 hours, or the police must get leave from a magistrate to keep him for longer.

Immediately after the hearing he was put into a police van and driven to the city’s Segerea maximum security prison.

Other accusations have been levelled at Kabendera in the last week. The day after his arrest a police spokesman said they were working with immigration officials to “clarify his citizenship”. He was then charged with sedition under cybercrime laws, controversial legislation that has been used by the president to jail his critics. Those charges were then also dropped.

Kabendera’s lawyers had been preparing for a bail application on Monday, but the crimes he was charged with are not bailable offences. He was not permitted to enter a plea because, according to the magistrate Agostino Rwezile, the court had no jurisdiction to hear his case.

The case was referred to the division of corruption and economic crimes of Tanzania’s high court and he will be held in Segerea until the next court date on 19 August.

“Our application was meant to grant the accused an opportunity to be heard in court or be granted bail but since he has been brought to court, we are asking that the charges be dropped,’’ said Kabendera’s lawyer, Jebra Kambole. Otherwise, he said, he had no objections. “We only ask the prosecution to speed up their investigation,” he said.

Press freedom campaigners called on prosecutors to drop the charges straight away and free him unconditionally.

“Initially police claimed to be questioning Erick Kabendera on his citizenship. Today they have brought to court drastically different, and clearly retaliatory, charges,” said Muthoki Mumo of the Committee to Protect Journalists. “The continued detention of this freelance journalist is an attempt to muzzle a critical voice and his case also has the potential to intimidate others in Tanzania’s media community into silence.”

The British high commissioner has spoken to Tanzanian officials about the case, and Kabendera’s supporters promised to fight for his freedom.

“We know you didn’t organise those crimes. We know you don’t evade tax,” Zitto Kabwe, an opposition politician, posted on his Instagram page. “It’s your journalism, your stance, your professionalism. We, your friends, will fight for your rights and we will win. You will be free.”