Willibrod Slaa, a former ambassador and opposition leader, the opposition activist Mdude Nyagali, and lawyer Boniface Mwabukusi were arrested over the weekend.

Mr Mwabukusi had led a court petition arguing that the deal undermined the constitution, national sovereignty and security. Mr Mwabukusi and Mr Nyagali were arrested after holding a news conference in Dar es Salaam criticising the port agreement.

All three men have been denied bail and were told they would be charged with treason, say their lawyers, which carries a mandatory death penalty. The authorities have not commented publicly on the charges.

Amnesty now says "the authorities must stop arbitrarily detaining activists simply for peacefully expressing their views", asking them to "immediately and unconditionally release these activists".