Zardad kept a "human dog" to savage his victims
Faryadi Zardad, 42, of Streatham, south London, was convicted in a retrial of pursuing a reign of fear at Afghan checkpoints between 1991 and 1996.
It is thought to be the first time a foreign national has been convicted in a UK court for crimes committed abroad.
The Afghan fighter, who had denied all charges, will be sentenced on Tuesday.
Victims traced
The warlord was said to have kept a "human dog" to savage his victims.
Zardad, who was in charge of the road between the Afghan capital Kabul and the city of Jalalabad, was first tracked down at his south London home by John Simpson for BBC Newsnight.
The programme was referred to Anti-Terrorist Branch officers to investigate after it was seen by MPs.
What followed was a lengthy police investigation in Britain and Afghanistan and court cases costing at least £3 million.
Officers, under armed escort, made several trips to Afghanistan and tracked down the warlord's victims.
The Old Bailey jury found Zardad guilty after hearing of numerous incidents of summary execution and hostage taking.
Zardad was retried after the jury in his first trial, last year, had been unable to agree.
One of the key legal challenges was to show that although Zardad did not necessarily administer torture himself he was still responsible through the men he controlled at his checkpoints.
'Human dog'
The government's top law officer, Attorney General Lord Goldsmith, came to the court for the first time since his appointment to prosecute the case.
He explained why Britain had decided to try the case, arguing that Zardad's crimes were so "merciless" and such "an affront to justice" that they could be tried in any country.
"Mr Zardad was found in England. An international convention and English law allow the trial in England of anyone who has committed torture or hostage-taking, irrespective of where those crimes were committed."
But Lord Goldsmith's number two in the first trial, James Lewis QC, took over for the second.
He said Zardad's men had used "indiscriminate and unwarranted violence on innocent civilian travellers.
"They would detain and imprison them. They would hold them for ransom or exchange civilians taken at the checkpoint elsewhere," said Mr Lewis.
The checkpoint was on the Kabul to Jalalabad road
Lord Goldsmith had alleged at the outset that Zaradad kept a man dubbed a "human dog" in a hole who would be set upon people, biting them and "eating testicles".
But no witnesses gave evidence of seeing the "dog" during the first trial.
Some eye witness evidence did emerge in the second trial, with one saying he saw a man being bitten by the long-haired "dog" at a checkpoint because he was too slow in dishing out fruit to the soldiers.
In both trials, evidence from Afghan witnesses - many in fear of their lives - was beamed into the British court via a video link from the UK embassy in Kabul.
One witness said he was held for four months and beaten so frequently that his family failed to recognise him.
But Anthony Jennings QC, for the defence, urged jurors to treat prosecution witnesses from Afghanistan with care and ask whether they had an axe to grind.
Zardad himself told the court he had not tortured anyone but had given orders against torture.
The Crown Prosecution Service said it was the first time in any country where offences of torture and hostage taking have been prosecuted in such circumstances.
"It is even more unusual for an English court to try matters when neither defendant nor witnesses are British subjects," the CPS said in a statement.
After the jurors returned their verdicts, the judge Mr Justice Treacy told them they had been involved in a "difficult and historic case".
Zardad came to Britain in 1998 on a fake passport and sought asylum.
But when he learnt he was being investigated in Afghanistan he dropped his application.
(also known as Zardad Khan and Commander Zardad), a warlord who controlled several checkpoints at the road between Kabul and  in Afghanistan in the early 1990s, was convicted today of torture and hostage-taking. The Court at  in London found that Zardad pursued a ''reign of fear'' along his checkpoints and ordered summary executions and torture of innocent civilians. He was sentenced to 20 years imprisonment.

Zardad denied all charges throughout the trials, and claimed he had been set up by enemies in Afghanistan.

"I have never killed anybody and if anybody in the whole of Afghanistan can produce that evidence against me then I will accept that crime," he said.

The case is considered to be a landmark in international law. British law allows the prosecution of crimes like torture and hostage-taking in the UK even if no British citizen was involved and the incidents occurred overseas. "There are some crimes which are so heinous, such an affront to justice, that they can be tried in any country", Attorney-General Lord Goldsmith said.

Zardad fled to the UK in 1998 after he had fought both the Russians and the Taliban in Afghanistan's long civil war. He settled in a suburban London home and was first confronted by BBC Journalist  in 2000. Simpson's report sparked a lengthy investigation that would eventually lead to British prosecutors travelling to Afghanistan and questioning Zardad's victims. A first trial began in 2004 but failed because the jury couldn't agree on a verdict.