A doctor granted British citizenship was a key organiser in the 1994 Rwanda genocide, a court has heard.
The men all deny charges of conspiring to kill Tutsis
Dr Vincent Bajinya is considered a "category one offender" by Rwandan prosecutors, the City of Westminster Magistrates' Court was told.
The 46-year-old Rwandan is one of four men facing extradition from Britain to Rwanda under a special diplomatic deal.
All four deny personal involvement in the slaughter of the Tutsi ethnic group and are fighting extradition.
Dr Bajinya, of Islington, north London, appeared alongside Charles Munyaneza, 49, a cleaner from Bedford, Celestin Ugirashebuja, 54, from Walton-on-the-Naze, Essex, and Emmanuel Nteziryayo, 45, of Manchester.
Each faces a range of charges, including genocide, crimes against humanity and participation in acts of devastation, massacres and looting.
District Judge Anthony Evans is presiding over the week-long extradition hearing in London.
Rwanda waived the death penalty to enable extradition from the UK.
James Lewis QC, representing the Rwandan government, opened the first full hearing of its kind to come before a British court.
After outlining the allegations against all four in turn, he repeated the line: "As a result of his actions, on his own and in concert with others, thousands of Tutsis were killed."
An orgy of violence erupted in the African country in 1994 after the Hutu president Juvenal Habyarimana's plane was shot down and ethnic Tutsis were blamed.
As soon as his death was announced, killing squads began attacking Tutsis across the country.
The court heard that Bajinya, who changed his name to Dr Vincent Brown by deed poll on becoming a British citizen last year, is accused of being a militia organiser in the capital Kigali at the time of the carnage.
The other three were allegedly bourgmestres - or mayors - of local communes in the country and are accused of organising and leading the killing in their areas.
Mr Lewis told the court that before the genocide, Dr Bajinya had been part of President Habyarimana's inner circle - or "akazu" meaning "little house".
He added that Dr Bajinya, a former member of the governing MRND party, was present at key "genocide meetings".
Mr Lewis told the court that he became a leader in the Interahamwe militia, which was to spearhead the slaughter.
Court papers allege he ordered the militia to cut a suspected Tutsi "into pieces so that he would not recover".
Dr Bajinya is also accused of personally interrogating a Tutsi woman about where her fellow "inyenzi" - or cockroaches - were before a militia man shot her dead.