Police used tear gas and water cannons to disperse violent demonstrators in Rome as tens of thousands of pro-Palestinian protesters took to the streets in major European cities and around the globe Saturday to call for a cease-fire in Gaza as the first anniversary of the Hamas attacks on Israel that started the war approached.

Massive rallies were planned over the weekend in several European cities, with gatherings expected to peak Monday, the date of the anniversary.

In Rome, several thousand demonstrated peacefully Saturday afternoon until a smaller group tried to push the rally toward the center of the city, in spite of a ban by local authorities who refused to authorize protests, citing security concerns.

Some protesters, dressed in black and with their faces covered, threw stones, bottles and paper bombs at the police, who responded with tear gas and water cannons, eventually dispersing the crowd. At least 30 law enforcement officers and three demonstrators were injured, local media reported.

The rally in Rome had been calm earlier, with people chanting, “Free Palestine, free Lebanon.”

Oct. 7 is the one-year anniversary of the Hamas militant group’s attack in southern Israel that triggered the war. The militants killed about 1,200 people, mostly civilians, and took some 250 hostage.

The ongoing war in Gaza has killed nearly 42,000 Palestinians, according to the Gaza Health Ministry. Its figures do not distinguish between combatants and civilians, but they indicate that more than half of the dead are women and children. Much of Gaza has been destroyed and about 90% of the enclave’s population has been displaced, many multiple times.

And the conflict between Israel and the Lebanese militant group Hezbollah has also expanded. The Iranian-backed Hezbollah started launching rockets into northern Israel on Oct. 8 and has traded fire with the Israeli military since. Israel recently increased its strikes in Lebanon, killing top Hezbollah leaders as well as civilians and displacing hundreds of thousands of Lebanese.

On Saturday in Paris, several thousand protesters gathered peacefully at Republique Plaza, holding Palestinian flags and posters reading, “Free Palestine” and “Hands off Lebanon.”

In London, pro-Palestinian protesters and counterdemonstrators, some holding Israeli flags, passed one another as thousands gathered in Russell Square. Scuffles broke out as police officers pushed back activists trying to get past a cordon. At least 17 people were arrested, London’s Metropolitan Police said.

In the northern German city of Hamburg, about 950 people staged a peaceful demonstration with many waving Palestinian and Lebanese flags or chanting, “Stop the genocide,” the DPA news agency reported, citing a count by police. Two smaller pro-Israel counterdemonstrations took place without incident, it said.

Pro-Palestinian demonstrators also gathered at New York’s Times Square to call for a cease-fire, chanting “Gaza!” to a drumbeat. Some wore kaffiyeh scarves, waved Palestinian and Lebanese flags and held a large cardboard image of Israeli Prime Minister Benjamin Netanyahu with red paint symbolizing blood across his face.

Rallies were also planned in several other cities in the United States as well as in other parts of the world, including Denmark, Switzerland, South Africa and India.

Pro-Israeli demonstrations are expected to be held Sunday because Jews across the world are still observing Rosh Hashana, or the Jewish New Year.

Security forces in several countries warned of heightened levels of alert in major cities, amid concerns that the escalating conflict in the Middle East could inspire new terrorist attacks in Europe or that the protests could turn violent.

Ben Jamal, director of the Palestine Solidarity Campaign in Britain, said he and others will keep organizing marches until action against Israel is taken.

“We need to be out on the streets in even bigger numbers to stop this carnage and stop Britain being drawn into it,” Jamal said.

Associated Press writers Zampano and Hui reported from Rome and London, respectively. AP writers Sylvie Corbet in Paris and Jamey Keaten in Geneva and Los Angeles Times staff contributed to this story.