Raza Husain, one of the lawyers defending migrants, told the Court of Appeal at the start of a four-day hearing that Rwanda was an authoritarian one-party state that did not tolerate opposition and imprisons, tortures and murders opponents.
The lawyers say the government's argument that Rwanda is a "safe third country" is flawed.
"There will only be any form of deterrent effect if a third country to which asylum seekers are removed is one to which they would not wish to go," Husain said, arguing the government had failed to tread the line between deterring migrants and remaining within Britain's human rights obligations.
However, lawyers representing the British government stressed that its deal with Rwanda is “subject to an exacting set of monitoring arrangements”, including by the UN High Commissioner for Refugees, who has intervened in the appeal.
They said in court filings that evidence about Rwanda’s asylum system was of little relevance as it had no bearing on how asylum seekers would be dealt with under the deal with Britain.
Prime Minister Rishi Sunak's government wants to send thousands of migrants more than 6,400 km away to Rwanda as part of a 140 million pound ($174 million) deal agreed last year.
It regards the plan as central to deterring record numbers of asylum seekers arriving in small boats across the Channel from France, something Sunak has made one of his five priorities, facing pressure from some of his own Conservative lawmakers and the public to resolve the issue.
In December, the High Court in London ruled the policy was lawful, but that decision is being challenged by asylum seekers from countries including Syria, Sudan, Iraq, Iran and Vietnam along with some human rights organizations.
Their lawyers say the government's argument that Rwanda is a "safe third country" is flawed.