* Kremlin has cracked down on opposition websites
* Ukraine authorities have suffered cyber attacks
* Kremlin denies link with events in Ukraine
By Timothy Heritage and Maria Tsvetkova
MOSCOW, March 14 (Reuters) - Victories are hard to come by for Vladimir Putin's opponents, activists are jailed, protests draw dwindling crowds, but on Friday they celebrated a minor triumph by briefly knocking out the Kremlin website.
To red faces in the Kremlin and government, the central bank's site was also brought down by a cyber attack and the Foreign Ministry suffered similar problems.
"A powerful cyber attack is under way on the (Kremlin) site," a spokeswoman for the Russian president's press service said by telephone as security experts struggled to curtail disruption. All three sites were working later on Friday.
A group calling itself Anonymous Russia highlighted the Kremlin website's crash on Twitter, signalling it may have been behind the attack. The same group said it brought down the website in May 2012 in solidarity with protests against Putin on his return for a third term as president.
A Kremlin source told Itar-Tass news agency there was no link with "the events in Ukraine", referring to the standoff with the West over Crimea, which votes on Sunday on unification with Russia.
But the cyber attack reveals a small chink in the Kremlin's defences as it defies Western demands to pull forces back to base on the Ukrainian Black Sea peninsula.
Critics may see Friday's attacks as revenge for similar attacks on official websites in Ukraine since the national parliament ousted Moscow-backed President Viktor Yanukovich last month and the standoff with Russia worsened.
Friday's attacks are particularly embarrassing for Putin because Russia blocked access to the Internet sites of prominent Kremlin critics Alexei Navalny and Garry Kasparov on Thursday, under a new law critics say is designed to silence dissent.
"In this case we are talking about 'hacktivism', a form of cyber attacks which is an expression of political or social protest," said Sergei Lozhkin, an expert at the Kaspersky Lab computer security firm.
"For someone who has certain skills it is much easier to attack the government website or the media than organise a real protest or demonstration."
He said there had also been similar attacks this week on the Internet sites of several Russian media outlets that support Putin, causing embarrassment although having no lasting impact on them.
The Kremlin denies allegations of censorship or pressure on the media but Thursday's move was the latest in what government opponents see as a crackdown on independent media and particularly the Internet.
The Internet is less easy to control than the state channels which dominate the airwaves in Russia and social websites have been used by Putin's opponents to summon people to protests against him.
Soon after Yanukovich's removal in Ukraine, the new authorities said the country's telecommunications system had come under cyber attack, with equipment installed in Russian-controlled Crimea used to interfere with mobile phones of members of parliament.
Unidentified sources later launched denial of service (DoS) attacks - intended to make a machine or network resource unavailable to its intended users - on Ukraine's top security body, the Security and Defence Council.