Vandals forced their way into the synagogue in Finsbury Park, north London, late Saturday and ransacked the building.
They threw prayer shawls and skull caps on the floor, stamped on an Israeli flag and splattered green paint on the Holy Ark, where sacred scrolls are kept.
"Tension in the Middle East leads to tension on the streets of Europe but this is one of the worst incidents of anti-Semitism we have seen in recent years," said Mike Whine, spokesman for the Community Security Trust, which monitors anti-Semitic attacks and intimidation.
Synagogues, Jewish schools and cemeteries have been targeted in several European nations in the past few weeks. In one of the most serious cases in France, a synagogue in the southern city of Marseilles was burned down on March 31.
Scotland Yard said police were treating the vandalism at the Finsbury Park District Synagogue as racially motivated.
Ian Ephraim, an officer for the synagogue, said the vandals broke in by pulling off a metal grille and smashing a window. About 15 interior windows also were shattered, and sand from a fire bucket was strewn across the floor.
Karen Pollock, director of the Holocaust Educational Trust, said: "The destructive and malicious nature of this act is sickening."
According to The Independent newspaper, some members of the Jewish community blame militant Muslims for the attack. The north London suburb is also home to Finsbury Park mosque, widely regarded as a center of radical Islam.
However, vandals left a British flag propped against the lectern, suggesting the attack could have been carried out by right-wing British nationalists.
"One option is it's the European right. Another option is Islamic fundamentalists," said Rabbi Saul Zneimer, chief executive of the umbrella organization United Synagogues. "We genuinely don't know."