Lawyers for Tanzania's jailed opposition leader Tundu Lissu filed a complaint on Friday to the United Nations Working Group on Arbitrary Detention in a bid to ramp up international pressure for his release.

Lissu, chairman of Tanzania's main opposition party and runner-up in the 2020 presidential election, was arrested last month and charged with treason, a capital offence, over comments he is alleged to have made calling on supporters to prevent national elections in October from going ahead.

Tanzania's government spokesperson did not immediately respond to a request for comment.

While President Samia Suluhu Hassan has won plaudits for easing political repression, she has faced questions about unexplained abductions of government critics in recent months.

Hassan, who will stand for re-election in October, has said her government respects human rights and ordered an investigation into the reported abductions.

Lissu's international lawyer, Robert Amsterdam, said the confidential complaint to the U.N. working group, which issues opinions but has no enforcement power, was part of a wider pressure campaign.

The European Parliament this month adopted a resolution denouncing Lissu's arrest as politically motivated, and Amsterdam said he would petition the U.S. State Department to impose sanctions.

"Right down to prosecutors, judges, police - all the people that are involved in this false show trial had better be aware that they should protect their U.S. assets," Amsterdam told Reuters.

In response to the European Parliament resolution, Tanzania's foreign ministry said outside criticisms about the case were based on "incomplete or partisan information".

The U.S. State Department did not immediately respond to a request for comment.

Lissu, who was shot 16 times in a 2017 attack for which no one has ever been charged, will appear in court on Monday.

Before he appeared in court last week, authorities detained a Kenyan and a Ugandan rights activist who had come to attend the hearing.

They were abandoned several days later near the borders of their home countries, and the Kenyan activist, Boniface Mwangi, said both were badly tortured while in custody.

Tanzanian officials have not responded to requests for comment about the allegation. Hassan has warned outsiders against "invading and interfering in our affairs".