Nearly 600 protesters, mostly women, were arrested on Thursday after they staged a non-violent action in the heart of a US Senate office building in Washington against Donald Trump’s “zero-tolerance” policy towards immigrants and separation of families at the border.

The mass protest was one of several demonstrations that erupted across the country, providing a taste of what are expected to be much larger demonstrations on Saturday called by the Women’s March and the Center for Popular Democracy Action. The rallies are likely to get a further boost as a result of the announcement on Wednesday by Anthony Kennedy that he is retiring from the US supreme court, providing Trump with the chance to make a second ultra-conservative appointment to the nation’s highest court and prompting fears of a rollback of liberal protections.

Facebook Twitter Pinterest Demonstrators in opposition to the immigration policies of the Trump administration rally at the Hart Senate office building. Photograph: Jonathan Ernst/Reuters

The largest demonstration on Thursday saw the 90ft atrium of the Hart Senate office building overrun by hundreds of women who sat on the floor pumping their fists in the air. Many were draped in foil sheets as a statement on the flimsy bedding given to children and adults as they are held at US border detention facilities.

The protesters chanted “abolish Ice” and “we care” – a jab at Melania Trump for wearing a jacket with the words “I really don’t care, do U?” painted on its back as she visited one of the immigrant detention centers in Texas last week.

Among those arrested in the Senate building were Pramila Jayapal, a Democratic member of Congress representing Seattle. She said in a video posted on social media that she had joined the action to protest against “the inhumane and cruel ‘zero-tolerance’ policy of Donald Trump and this administration, the separation of families, the caging of children, and the imprisonment of asylum seekers”.

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She added that she was proud to have been arrested among hundreds of others who believed that “the US is better and as a member of Congress I refuse to let this president and administration do what they are doing to children in my name”.

Facebook Twitter Pinterest Senator Elizabeth Warren cheers on demonstrators. Photograph: Jonathan Ernst/Reuters

Daily images are still emerging of distraught immigrant children separated from parents and not yet reunited, despite an executive order last week ending the policy of summarily tearing families apart and arresting the adults after anyone is caught crossing the border illegally.

In a written statement, the Capitol police said about 575 people were charged with unlawfully demonstrating inside the office building. The police said those arrested were being released after they were processed.

Winnie Wong, political adviser for the Women’s March, said the crowd’s fervor would translate into “the energy we will need to see at the ballot box in November”, when congressional control will be at stake.

Facebook Twitter Pinterest The actor Susan Sarandon, center, was among the activists protesting at the Hart office building. Photograph: J Scott Applewhite/AP

The Democratic senator Jeff Merkley of Oregon, who was one of the first to draw attention to children being held in detention camps along the border after being separated from their parents, appeared before the crowd. So did the Democrats Kirsten Gillibrand of New York and Edward Markey of Massachusetts.

“These folks are out here fighting for the core principles of our nation, and I applaud them for it,” Merkley said in an interview.

Meanwhile, hundreds more people gathered at a rally outside a federal courthouse in Brownsville, Texas, in the Rio Grande valley.

And dozens of protesters shut down a government meeting in Michigan in protest against a contract with the Immigration and Customs Enforcement (Ice) agency to house detainees at a local jail. Eight people also were arrested outside an Ice building in Portland, Oregon, that has been closed because of a round-the-clock demonstration.