MOSCOW — Britain’s ambassador to Moscow yesterday directly linked the Kremlin to a campaign of harassment waged against him by an ultra-nationalist youth movement.

Anthony Brenton said he had been the victim of four months of coordinated intimidation by the Nashi youth movement, an organization that has pledged loyalty to President Putin.

“Nashi’s links with the Kremlin are well enough known,” he said. “Their leader has met with President Putin many times, and one of his advisers was known to have been involved in its creation. Even if one were to accept that they are not directly controlled by the Kremlin, this level of influence suggests that the Kremlin could stop them if it wanted to.”

The movement has obtained copies of Mr. Breton’s daily diary — something that could suggest the involvement of the FSB spy agency — and used it to trail the ambassador wherever he goes. Nashi youths have staked out his home and the embassy. They follow him, block his car on occasions, and disrupt meetings. At one lunch, heckling youths rocked his chair, raising fears that he would be assaulted.

Mr. Brenton said he could not go shopping without facing a barrage of abuse. “When I go out of the house to buy cat food, they follow me and start waving banners,” he said.

Ostensibly at least, Nashi’s campaign stems from Mr. Brenton’s attendance at a summit convened by Russia’s liberal opposition in July to protest the limits imposed on civil society by the Kremlin. Nashi says that Mr. Brenton participated in a “fascist meeting” and promised to hound him until he apologized for attending.

Mr. Brenton’s speech also infuriated Mr. Putin. Britain, which has emerged as Russia’s fiercest critic within the European Union, has particularly irked the Kremlin.

The Russian government’s reaction to repeated complaints over Nashi’s actions from the embassy showed how low Britain’s stock has fallen, even before differences emerged following the murder of ex-KGB officer Alexander Litvinenko.

Although Russia is a signatory to the Vienna Protocols, which require host countries to ensure the safety of diplomats, the Foreign Ministry initially insisted that Nashi’s actions were “not illegal.” Even when they later agreed to act, Nashi’s campaign has continued unabated.

Created last year largely by the deputy chief of the presidential staff, Vladislav Surkov, Nashi has become a useful tool for crushing dissent. Nashi youths have infiltrated opposition movements, beaten up activists, and held massive demonstrations.

Other tools have been used to target other British interests in Russia. The British Council in St. Petersburg suffered repeated tax inspections earlier in the year and is now being threatened with closure by the fire safety department and the British Broadcasting Corp.’s Russia service has been taken off the air several times in the past year. Russia’s problem with Britain essentially stems from court decisions granting political asylum to two of Mr. Putin’s least favorite people: the oligarch who helped bring the president to power but then turned against him, Boris Berezovsky, and a Chechen rebel envoy, Ahmed Zakayev.