Global rights watchdogs are accusing the government of Tanzania of a crackdown on critics of the country’s controversial port deal with a Dubai firm.

Amnesty International and the Human Rights Watch (HRW) say authorities have arrested critics of the deal, which some opponents say it places Tanzania on the losing end.

And the rights watchers say the country has turned to Magufuli-era suppression of dissent to silence mounting opposition to the deal given to DP World, an Emirati logistics firm.

Willibrod Slaa, a former opposition politician who later served as Tanzania's ambassador to Sweden under ex-president John Magufuli, on Sunday became the highest-profile personality to be detained by police over the controversial agreement that has polarised public opinion over its possible implications for Tanzania's control of its ports.

Mr Slaa, lawyer Boniface Mwabukusi and Mdude Nyagali, a political activist, are still behind bars as police launched a fresh crackdown on critics of the agreement recently given backing by the High Court on Thursday last week.

Read: Tanzania court gives hope for Dar port deal with DP World

In a statement issued late Monday, Amnesty called on Tanzanian authorities to release all three "immediately and unconditionally" and "stop arbitrarily detaining activists simply for peacefully expressing their views (in) respect of the right to freedom of expression."

“Tanzanian authorities’ crackdown on critics of the UAE port deal reveals their growing intolerance for dissent," said Tigere Chagutah, Amnesty International’s Regional Director for East and Southern Africa.

The Inter-Governmental Agreement (IGA) signed between Tanzania and the UAE in October last year is set to allow Dubai-based multinational logistics company DP World to take over operations at the port of Dar es Salaam to start with and later other sea and lake ports in the country, depending on negotiations. 

It was approved by parliament on June 10 this year but has continued to be met with harsh criticism from various actors including opposition politicians, legal experts and religious clerics who believe that it will benefit the Dubai firm more than Tanzania.

Read: Port deal outrage sends Samia, CCM to early campaigns

In its August 10 ruling on a petition filed by four private citizens that challenged the legality of the pact, Tanzania’s High Court sitting in Mbeya said it lacked merit almost totally and upheld the government's argument that the IGA was merely a "framework" document for future binding contracts that would provide more clarity on the scope of DP World's ports involvement, possible limits and Tanzania’s own expected benefits. 

Mr Mwabukusi, who represented the petitioners in the case, said after the ruling that they intended to file an appeal at the Court of Appeal. Speaking in the heat of the moment, he also suggested that they might also "take the matter to the streets."

According to Amnesty International, he and Mr Nyagali were arrested in Morogoro region on Saturday August 12 while on their way back to Dar es Salaam from Mbeya and by Monday were being held at the Central Police Station in Mbeya.

Amnesty also quoted Mr Slaa's lawyer as saying he was arrested on Sunday at his home in Dar es Salaam and taken to Oysterbay Police Station in the city after police conducted a search of the house and confiscated some communication devices. 

On Friday August 11 - a day after the High Court ruling - Tanzania’s Inspector General of Police Camilius Wambura said unnamed critics of the DP World deal would be apprehended over their “seditious” statements calling for "nationwide protests”.

According to IGP Wambura, the statements were also treasonous and tantamount to “inciting the public into overthrowing the government."

Under Tanzanian law, the offence of treason carries a mandatory death penalty and is also unbailable.

In an earlier August 7 statement, HRW said at least 22 people had been "detained or threatened" in Tanzania over the DP World accord since it was ratified by parliament on June 10.

Read: EYAKUZE: If you don’t feel port pact, please stand

It urged Tanzanian authorities to "stop harassing and arresting critics of the agreement" adding that such "suppression" was a "troubling sign of low tolerance for dissenting views.”

Among the instances that Human Rights Watch cited were earlier arrests of Mr Mwabukusi and Mr Nyagali (in July) plus two other lawyers, Peter Madeleka and former Tanganyika Law Society president Rugemeleza Nshala. Each of them has on different occasions been particularly outspoken against the deal on social media.