Washington, DC – The rotunda of an office building in the United States Capitol complex filled in minutes with hundreds of people who appeared as if from nowhere, as part of a surprise protest against Israel’s war in Gaza.

“Let Gaza live,” they chanted in unison inside the Cannon House Office Building, removing their outer garments to reveal matching red T-shirts.

One side of the shirt read, “Jews say stop arming Israel.” The other side: “Not in our name.”

Tuesday’s coordinated protest, led by Jewish Voice for Peace (JVP), took Congressional staffers and law enforcement agents aback, as it came together within minutes.

But the crackdown started almost immediately after the protesters assembled. Capitol Police quickly declared the area a closed “zone” and arrested hundreds of demonstrators who refused to leave.

“This is a moment in history where we have to say we stood up for Palestinian freedom. We stood up to end this genocide,” said protester Liv Kunins-Berkowitz.

“For so many of us, we are the descendants who survived ethnic cleansing and genocide. Our ancestors and grandparents taught us that the worst thing to do in these moments is to be a bystander.”

Kunins-Berkowitz added that the protest is part of a tradition of peaceful civil disobedience. “It’s what we have to do when our government refuses to listen to the people,” she told Al Jazeera.

The demonstration on Capitol Hill comes a day before Israeli Prime Minister Benjamin Netanyahu is set to deliver a speech to Congress at the invitation of top legislators from both major parties.

Netanyahu’s government has overseen more than nine months of death and devastation in Gaza, with experts and rights advocates warning of “genocide” in the Palestinian territory.

Since the war began, the Israeli offensive has killed more than 39,000 Palestinians, many of them women and children.

Nevertheless, the US government has remained uncompromising with its support for Israel.

The administration of President Joe Biden has authorised more than $14bn in military aid to Israel, while also maintaining a steady flow of weapons and bombs.

In addition, the US has vetoed three United Nations Security Council resolutions that would have called for a ceasefire.

Biden called for an end to the war in May as part of a deal that would see the release of Israeli captives in Gaza, but he has pressed on with support for Israel, as the country’s leaders pledge to continue to fight until “total victory”.

Abby Stein, a rabbi and activist, said that while US officials are paying “lip service” to the ceasefire, protesters are calling for an actual end to the war and an arms embargo against Israel.

“I know that none of us will be safe while the US sends billions of dollars in weapons to Israel,” Stein told Al Jazeera.

“This is one of the worst atrocities we are watching unfold before our eyes. We are here today to make sure that we do everything we can to stop this.”

Progressive Jewish groups have been organising protests across the country, including a similar demonstration on Capitol Hill in October last year, as well as rallies on roads, inside train stations and at political offices.

Asked whether such direct action is effective, Stein was unequivocal: “Yes.”

She argued that protests raise awareness about the situation in Gaza and give exposure to the demands of the demonstrators.

“I think it definitely helps,” Stein added. “The action that happened in October is a good example. It really put us in the media to show that there are tens of thousands of Jews who are opposing what the Israeli government is doing.”

Activist Tal Frieden said that, as the grandchild of Holocaust survivors, he grew up hearing stories about the importance of ensuring that genocide does not happen again. “I’m here today to demand that the US stop sending weapons to Israel,” he told Al Jazeera.

At the protest on Tuesday, demonstrators held firm and refused to leave as their fellow activists faced arrest.

Law enforcement agents moved in quickly and discarded banners decrying the “genocide” in Gaza.

Then, they started arresting people from the outer edges, making the circle of activists smaller, as if peeling it layer by layer.

For more than an hour, officers steadily detained the activists and led them to an elevator away from the rotunda and to the underground halls of Congress.

Many zip-tied demonstrators continued to chant, “Stop arming Israel” and “Free, free Palestine”, as they were being taken away. But the chants grew fainter and fainter as the last demonstrators were taken out.

“We are arresting a group that is illegally demonstrating inside the Cannon Rotunda. Demonstrations are not allowed inside the Congressional Buildings,” Capitol Police said in a statement.

“We told the people, who legally entered, to stop or they would be arrested. They did not stop, so we are arresting them.”

A JVP organiser told Al Jazeera that the group was told the protesters “are being processed and released on their own recognizance with a post and forfeit”, suggesting they will not face charges.

JVP said 400 people were arrested, but Capitol Police has not released official figures.

“For nine months, we’ve watched in horror as the Israeli government has carried out a genocide, armed and funded by the US Congress and the Biden administration have the power to end this horror today,” JVP executive director Stefanie Fox said in a statement.

“Instead, our president is preparing to meet with Netanyahu and Congressional leadership has honored him with an invitation to address Congress. Enough is enough. Biden and Congress must listen to the people: We need an arms embargo now to save lives.”