Four men accused of taking part in the Rwanda genocide in 1994 have been remanded in custody by UK magistrates.
The men all deny charges of conspiring to kill Tutsis
Vincent Bajinya was arrested in north London, Charles Munyaneza in Bedford, Celestin Ugirashebuja in Essex and Emmanuel Nteziryayo in Manchester.
The four appeared at City of Westminster Magistrates Court after an extradition request from Rwanda.
They all deny charges of conspiring to kill the Tutsi ethnic group and aiding and abetting the mass killings.
Gemma Lindfield, acting for the Rwandan government, gave the court background to the events of 1994 in which some 800,000 Tutsis and Hutu moderates were killed.
When then President Juvenal Habyarimana died in a plane crash, Hutu militias were set up across the country and given plans to kill Tutsi neighbours, she said.
The court heard that Mr Munyaneza, 48, Mr Ugirashebuja, 53 and Mr Nteziryayo, 44, were local mayors with sweeping powers in their areas.
Miss Lindfield told the court Mr Munyaneza urged Tutsis to flee to safety in public buildings, where Hutu militias were lying in wait to kill them.
"It is estimated that there were tens of thousands of Tutsis killed in his district and as bourgmestre [the local mayor] he was very much responsible for the planning of the killings," she added.
She said Mr Ugirashebuja's role in his area had been to "make sure the Tutsis were being killed".
"He also organised road blocks in the commune to prevent the escape of Tutsis and again is responsible for many thousands of Tutsi lives," she added.
Mr Bajinya, 45, was a militia co-ordinator who had led militiamen on searches of houses of Tutsis to kill them in their own homes, Miss Lindfield said.
Those who escaped were murdered at roadblocks which he had personally helped to man, Miss Lindfield said.
Because there was no application for bail for Mr Nteziryayo, no specific allegations against him were read out in court.
Bail applications for the other three men were refused.
The court also heard that Mr Bajinya had taken British nationality and changed his named to Vincent Brown.
Both Mr Ugirashebuja and Mr Munyaneza had previously been granted refugee status which had now been cancelled by the Immigration and Nationality Directorate, Miss Lindfield said.
No details were given of Mr Nteziryayo's immigration status.
An extradition warrant accuses the four of murdering and aiding and abetting the killing of Tutsis between 1 January 1994 and 12 December 1994.
Under an agreement between the two countries, Rwanda has agreed that the men will not be given the death penalty if they are convicted, the court heard.
A remand hearing for all four defendants is due to take place on 5 January, although they are not expected to appear in person before the court again until 26 January.