Three men accused of taking part in the Rwanda genocide in 1994 have launched an attempt in the High Court to block extradition proceedings.
The men all deny charges of conspiring to kill Tutsis
Vincent Bajinya was arrested in north London, Celestin Ugirashebuja in Essex and Emmanuel Nteziryayo in Manchester.
The case comes after Rwanda waived the death penalty to become a temporary extradition partner with the UK.
The men's lawyers argue the government was wrong to allow Rwanda more time than usual to submit documents.
The men deny charges of conspiring to kill the Tutsi ethnic group and aiding and abetting the mass killings.
Mr Ugirashebuja, 53 and Mr Nteziryayo, 44, were said by a Rwandan government prosecutor to have been local mayors and Mr Bajinya, 45, is alleged to be a former militia co-ordinator.
Rwanda has no permanent extradition treaty with UK. But the temporary arrangement was set up after it agreed the men will not face the death penalty if extradited and convicted.
The 2003 Extradition Act, covering those countries where permanent agreements are in force, requires that requests detailing evidence against suspects be filed within 45 days of an arrest.
The time Rwanda was allowed to submit documents containing evidence of a case against the men had been extended.
Defence barrister Alun Jones QC contended the policy should not be overridden.
Home Secretary John Reid had no power to dispense with basic requirements "sacrosanct" in law, he said.
Judgment on the challenge was reserved.