The U.S. Holocaust Memorial Museum expressed alarm on Monday over "hateful speech" at a white nationalist meeting over the weekend, and a restaurant apologized for hosting the group after a woman tweeted a picture of herself in a Nazi salute.

The National Policy Institute, a think-tank that is part of the alt-right movement that includes neo-Nazis, white supremacists and anti-Semites, held a gathering in Washington, D.C., at the federally owned Ronald Reagan Building on Saturday.

A video by the Atlantic that was taken inside the conference showed Richard Spencer, leader of the National Policy Institute, shouting to a crowd of about 200 people: "Hail Trump, hail our people, hail victory!" as some attendees lifted their hands in a Nazi salute.

Spencer also called mainstream media "luegenpresse," German for lying press, a phrase regularly used during the Nazi era in Germany.

"America was until this past generation a white country designed for ourselves and our posterity," Spencer also says. "It is our creation, it is our inheritance and it belongs to us."

The National Policy Institute gathering drew scores of protesters who blocked traffic around the building to express their outrage over the meeting.

'I condemn them,' says Trump

The alt-right movement came to the fore during the U.S. presidential election. President-elect Donald Trump has drawn criticism for naming Steve Bannon, former head of a website linked to the alt-right, as his chief White House strategist.

Trump condemned the National Policy Institute in an interview with New York Times editors on Tuesday.

U.S. president-elect Donald Trump reacts to a crowd gathered in the lobby of the New York Times building after a meeting on Tuesday. He told the reporters he condemns the alt-right think-tank whose members praised him with Nazi salutes over the weekend. (Lucas Jackson/Reuters)

"I condemn them. I disavow, and I condemn," he said. "It's not a group I want to energize. And if they are energized I want to look into it and find out why."

He also defended Bannon, saying: "I've known Steve Bannon a long time. If I thought he was a racist, or alt-right … I wouldn't even think about hiring him."

Holocaust 'began with words'

"The Holocaust did not begin with killings: it began with words," the Holocaust Memorial Museum said in a statement. "The museum calls on all American citizens, our religious and civic leaders and the leadership of all branches of the government to confront racist thinking and divisive hateful speech."

Some of those who attended the nationalist meeting later gathered for a banquet at Maggiano's Little Italy restaurant in northwest Washington. Protesters converged on the site and prompted the restaurant to close to protect the safety of staff and guests, the management said in an apology on Facebook.

The restaurant said the booking was a last-minute affair made under a different name, so staff was unaware the National Policy Institute was dining there and did not know what the group represented.

"After the event, an attendee sent a tweet in which she made a 'Sieg Heil salute' in support of Hitler and white supremacy.

"This expression of support of Hitler is extremely offensive to us, as our restaurant is home to teammates and guests of every race, religion and cultural background," the Facebook post said.

The restaurant apologized to the surrounding neighbourhood and said it was donating the profits from its sales on Friday, $10,000 US, to the Washington office of the Anti-Defamation League, a Jewish civil rights group.

Ronald Reagan Building defends hosting group

The Ronald Reagan Building, the biggest structure in the U.S. capital city, is federally owned, but run by Trade Center Management Associates, a unit of Drew Co., a Boston real estate development company.

The firm defended allowing the nationalist group to use the venue. It said in a statement that its contract mandates that unless there is a security threat by an organization or person who asks for meeting space, the building is available for rental.

The agenda of the National Policy Institute's Saturday meeting included a breakdown of the 2016 election and plans for the growth of the alt-right.

The nonprofit Southern Poverty Law Center, which tracks hate groups, described Spencer as an "academic racist" who backs creation of an Aryan homeland.

Last week, Twitter suspended Spencer's and several other accounts linked to the alt-right.

The move followed the microblogging service's announcement that it would upgrade some features to better combat cyberbullying.