Opponents of Syrian President Bashar al-Assad pelted the Russian Embassy in London with stones overnight, causing significant damage to the building, the embassy said in a statement on Friday.

“On the night of August 16-17, the consulate department of the Russian Embassy in London was attacked by a large group of people who were chanting slogans against the Syrian government and throwing stones at the building hosting the diplomatic mission,” the statement said.

The building had several window glasses shattered and suffered “significant material damage,” it said. None of the embassy employees was hurt.

Russian diplomats expressed regret over the conduct of London police who they said had arrived at the scene but had “failed to take proper measures to neutralize the unsanctioned demonstration which was under way and detain the attackers.”

The attack was not the first act of vandalism against the Russian embassy in London, the statement said. Earlier this year, supporters of the Syrian opposition splattered paint on the embassy fence, it said, adding that such attacks were violating the principle of immunity of diplomatic missions.

The Russian Foreign Ministry summoned the British ambassador to Russia, Tim Barrow, to express protest over the incident on Friday.

Russia, along with China, has vetoed three Western-backed UN Security Council resolutions against Assad, citing a pro-rebel bias.

Western powers have accused Russia of backing Assad, which Moscow has repeatedly denied, saying that Syrians themselves should decide whether he should stay in power or leave.

At least 17,000 people have been killed in Syria since the beginning of an anti-Assad uprising in March 2011, according to UN representatives quoting estimates by Syrian activists. Some 150,000 people who have fled Syria since the outbreak of the unrest have been officially registered as refugees, while observers say that the real number of refugees in four countries neighboring Syria (Lebanon, Jordan, Turkey and Iraq) could exceed 200,000.