Up to 20 more people suspected of leading a genocide in Rwanda that claimed one million lives are at liberty in Britain, the Guardian has learned.

The revelation comes after four Rwandans appeared in a London court yesterday accused of orchestrating one of the worst atrocities of the 20th century. They were arrested on Thursday evening after Rwanda had requested their extradition to stand trial for the mass murder which convulsed the African state in 1994.

The four were remanded in custody by City of Westminster magistrates; the extradition process is expected to be protracted, at least a full year.

British government sources say that more Hutus suspected of leading the mass murderer of Tutsis, while the rest of the world looked on, have slipped into Britain and are living here. A Foreign Office source told the Guardian the four were "the tip of the iceberg", and 20 or so more suspects were in exile in the UK. British officials say they cannot act until Rwanda makes a formal request, but the war-ravaged country has few resources to pursue suspected killers around the world.

Britain acted over the four after repeated requests from Rwanda and growing anger in the capital, Kigali, that the UK was apparently willing to give refuge to alleged mass murderers. Charles Munyaneza, 48, Celestin Ugirashebuja, 53, Emmanuel Nteziryayo, 44, and Dr Vincent Brown, 45, were arrested on Thursday night at their homes around Britain.

Rwanda has promised that, if they are convicted, they will not be executed; Britain will not extradite suspects to face the death penalty. Outlining the case to the magistrates, Gemma Lindfield, representing the Republic of Rwanda, said that three of the four were bourgmestres, or mayors, who allegedly used their local authority to carry out genocide.

Ms Lindfield said Mr Munyaneza set up militias to murder, and made speeches: "He spread the message that the Tutsis were to be wiped out." He told Tutsis to take refuge where Hutu killers were waiting for them, the court heard: "It is estimated there were tens of thousands of killings in his district."

Militias in Mr Ugirashebuja's area used machetes to hack victims to death, and also raped women, the court heard; and he organised roadblocks to stop any escape. Dr Brown commanded a militia in house to house searches of Kigali for Tutsis to butcher. Mr Nteziryayo was also a bourgmestre. "They were the organisers," Ms Lindfield said, "the facilitators of the genocide, in which hundreds of thousands of men, women and children were killed because they were Tutsis."

District judge Anthony Evans remanded the four in custody, saying there was a danger of them trying to flee. Through lawyers the four all denied the accusations. All four claimed asylum on entering Britain, but two at least were unsuccessful. They were identified by other Rwandans living in Britain.

The arrests followed months of work. Rwanda requested extradition in late summer, and, after discussion, home secretary John Reid signed extraordinary extradition warrants in October.

But no action was taken until Crown Prosecution Service lawyers had gone to Rwanda to check that indictments by prosecutors in Kigali were of sufficient standard to present to a British court; two retired UK police officers spent more than eight weeks interviewing witnesses cited in the indictments. And last week a memorandum of understanding was signed between Britain and Rwanda, because the two have no extradition agreement.

Though pleased Britain had acted, Tharcisse Karugarama, the justice minister, said yesterday: "We were angry and surprised the four were in the UK. We think it the responsibility of all states to ensure people accused of such crimes are not living freely under their protection. Why did Britain not check their backgrounds when they arrived?"