The Financial Times reports on the rampage against British diplomats by the Kremlin's crazed youth cult "Nashi" ("We Russian Slavs") in the wake of the Litvinenko killing.

They don't look too dangerous in the propaganda photo, do they? Stalin looked quite dapper in his moustache too, and remember the perils of the mermaid's siren song! But read on, dear reader, read on . . . let the facts speak for themselves.

It was shortly after Anthony Brenton, Britain's ambassador to Moscow, spoke about the challenges to freedom in Russia that a young blond man thrust himself forward and started to yell at the top of his voice: "Brenton, apologise!"

The stunt this week at the Humanities University in Moscow - where Mr Brenton was speaking alongside Sir Tom Stoppard, the British playwright whose trilogy about 19th-century Russian thinkers is being staged in Moscow - was not a one-off prank by an attention-seeking youth. It was part of a well-organised harassment campaign against the ambassador, apparently waged with the knowledge of the Kremlin, that is a striking symptom of the worsening relationship between Moscow and London.

The young man was a member of Nashi ("Our Own"): a Kremlin-sponsored youth movement with a well-earned reputation for thuggery. The organisation, which claims to have 7,000 active members and another 8,000 sympathisers has been stalking the UK ambassador seven days a week for the past four months, putting him and his family under considerable strain. "It is a deliberate psychological harassment which is done professionally and which borders on violence," says Mr Brenton.

Robert Shlegel, of Nashi, whose leaders regularly meet President Vladimir Putin, says this is retribution for Mr Brenton. The British ambassador was "guilty" of speaking at a conference of political opposition parties in Moscow shortly before the summit of Group of Eight leading industrialised countries in St Petersburg in the summer. Igor Shuvalov, an aide to Mr Putin, warned that attending the event - as several western ambassadors did - would be interpreted as an unfriendly act. No sooner had the G8 leaders left Russia than Nashi began its campaign, demanding an apology from the UK ambassador for "endorsing fascists".

The Nashi case is one part of the souring in Anglo-Russian relations. London's granting in 2003 of political asylum to Boris Berezovsky, the renegade oligarch, and Akhmed Zakayev, a Chechen opposition leader, infuriated Moscow. British Council offices in Moscow were raided in response and UK diplomats publicly accused of espionage. Images of a radio transmitter disguised as a rock allegedly planted by British spies were later broadcast by Russian state television.

The British investigation of the poisoning of Alexander Litvinenko, the former KGB officer, is likely to strain the relationship further. Spy rows and expulsions featured regularly in Anglo-Russian relations in Soviet times. But former British diplomats say that even the communists did not use personal intimidation and harassment of ambassadors and their families. Nashi members follow Mr Brenton with banners at weekends, shout abuse at him, block his car and advertise all his movements on the internet.

Earlier this month a group of Nashi supporters followed Mr Brenton to another city, disrupting a seminar where he spoke. "I was really worried they were going to hurt him," the organiser of the seminar said. The British embassy has officially complained to the Russian foreign ministry, pointing out that Moscow is breaking the Vienna convention on diplomatic relations, which demands that foreign envoys be treated with "due respect" and shielded from attacks on their "person, freedom and dignity".

Foreign ministry officials privately agree that Nashi's behaviour is outrageous and goes well beyond peaceful protest. But they claim there is little they can do: Nashi is too close to the Kremlin. It has close ties with Vladislav Surkov, the deputy head of the Kremlin administration, as well as Mr Putin himself. Nashi said it supported the Kremlin line, met Mr Surkov regularly and received funding from "large, nationally oriented companies". Its main task was to prevent the spread of "colour revolutions" of the kind that swept Ukraine and Georgia.

The word Nashi has a rich and poignant history, featuring in the novels of Dostoevsky. It was used in the early 1990s by Alexander Nevzorov, a crusading TV presenter, to extol Russian paratroopers trying to suppress popular uprisings in the Baltic states. Nashi claims that it is building a civil society in Russia and fighting against fascism. However, as Sir Tom put it, for people like these, "unpalatable argument is fascism - interrupting the speaker is the exercise of free speech".