A bottle of flammable liquid was hurled at one of Chicago's oldest synagogues, catching fire but not causing major damage.
No one was injured in the incident early Monday at Temple Sholom of Chicago.
Chicago police and the Chicago fire department are investigating the arson as a hate crime. No one was in custody Monday.
Police officer Daniel O'Brien says the fire burned itself out and never ignited the North Side building. He says investigators are working to get surveillance equipment from the area.
Roger Rudich, president of the temple, says the arson was unsettling but not damaging.
Officials say they don't know if there's a link between the incident and increased violence in the Middle East.
Tens of thousands of people throughout the Arab world and Europe protested on Monday against the IDF's Operation Cast Lead in the Gaza Strip.
In the Greek capital, Athens, 3,000 Greeks and Arabs protested outside the Israeli embassy. Police were forced to use teargas and shock grenades to control the demonstrators, some of which were throwing stones at the building.
One man was arrested after he managed to get through the police barricades and remove the Israeli flag from its post.
Outside the Israeli embassy in London, England, some 1,000 people protested against the Israeli attack for the second day in a row. Demonstrators carried signs supporting residents of Gaza. About a dozen protesters were arrested.
A number of anti-Israel protests were also held in France, in Paris, Toulouse, Montpellier and Marseille. A total of some 2,000 people took part in the French protests, and shouted, "Israel is a killer, Sarkozy is a collaborator".
Smaller demonstrations were held in Swedish capital Stokholm, Warsaw, Poland, and Helsinki, the capital of Finland.