Zambian immigration authorities on Friday blocked former president Rupiah Banda from leaving the country for the third time in nearly two months, sparking criticism from activists.
Banda, who is fighting corruption charges, was stopped from boarding a flight to South Africa despite a High Court order releasing his passport.
"We did all the airport formalities but surprisingly an immigration officer came and said he had instructions from above not to allow president Banda to travel," his press aide Kennedy Limwanya told AFP.
"We showed him a court order but he still said he could not allow us."
Banda was set to fly to South Africa on a morning flight for a conference, after the High Court ruling on Thursday.
His planned trip has been marred by hiccups after a magistrate's court refused to return his passport, which he had to surrender after his re-arrest in April on corruption charges.
The government Friday said it had slapped Banda with a formal travel ban.
"The investigations team has imposed travel embargoes against the former president for fear that he might interfere with the witnesses," said Home Affairs Minister Edgar Lungu.
Rights groups Friday strongly condemned the government's refusal to allow Banda's travels.
"Banda is a former head of state and he followed the normal channels to get his passport," said Bishop John Mambo of the Bible Gospel Church in Africa.
The consortium of 11 organisations said Banda was not a flight risk.
"He is not under house arrest. Let him be allowed to travel," said Mambo, a vocal critic of the Sata administration.
"Rupiah can't run away from his country... It's like he has been convicted even before the courts do so," he said.
Banda lost to current President Michael Sata in 2011 elections after a three-year stint in power.
The international community lauded the country's peaceful handover of power after Banda's party ruled for 11 years.
But soon after taking office Sata launched a raft of corruption cases against various officials of the former government.
Banda himself was stripped of diplomatic immunity in February and has been appearing in court on several corruption charges.
He was stopped from travelling to Kenya on April 9 to attend Uhuru Kenyatta's presidential inauguration, and last month was blocked from travelling to Ethiopia for an African Union summit.
"President Sata should know that what he is doing is a vicious cycle. One day he will be out of office and he will be subjected to similar humiliation," said John Mambo a cleric critical of Sata's government.
Zambia has a history of prosecuting its former leaders. Frank Chiluba governed from 1991 to 2002 and was prosecuted for corruption a year after leaving office.