Senior CHADEMA party officials, his lawyers and family members said they tried unsuccessfully on different occasions on Friday to get access to him at a jail in the capital Dar es Salaam where he has been held since April 9.
The Prisons Service denied Lissu had been moved from jail.
"Such information is misinformation and false. We would like to inform the public that Tundu Lissu is safe and he is still detained at Keko Prison in Dar es Salaam according to the country's laws and procedures," Elizabeth Mbezi, Tanzania Prisons Service spokeswoman, said in a statement.
Government spokesman Gerson Msigwa did not immediately respond to calls and text messages seeking comment.
"CHADEMA would like the Prisons Service, concerned government agencies to give information on where Lissu has been taken," the party said in a statement.
Lissu, runner-up in the country's 2020 presidential election, was charged with treason last week over what prosecutors said was a speech calling on the public to launch a rebellion and disrupt the election.
He was not allowed to enter a plea on the treason charge.
The charges against him will bring fresh scrutiny to President Samia Suluhu Hassan's human rights record as she bids for re-election later this year.
Last weekend, the election commission said CHADEMA would be disqualified from elections due in October over its refusal to sign a code of conduct as it demands electoral reforms.
Hassan won praise after coming to power in 2021 for easing repression of political opponents and censorship of the media that proliferated under her predecessor John Magufuli, who died in office.
But she has faced mounting criticism from human rights activists over a series of arrests and unexplained abductions and killings of political opponents.
Hassan has said the government is committed to respecting human rights and she ordered an investigation into reported abductions last year.
