Police Wednesday morning arrested about a dozen protesters in downtown Pittsburgh after they blocked the Fort Pitt Bridge off ramp by kneeling across Liberty Avenue at Commonwealth Place as part of a peaceful protest denouncing white nationalism.

The act of civil disobedience and a rally that drew scores of people outside the Gateway T station were organized by Bend the Arc Jewish Action, a group advocating progressive social change, hours before President Donald Trump is scheduled to arrive to address the shale conference.

Protesters carried gold signs featuring the Star of David and black lettering that read: “Our Solidarity will defeat white nationalism.” Those arrested, who ranged in age from the mid-30s to mid-60s, held a large white banner with the same slogan.

Police officers wearing riot helmets and carrying batons asked the protesters to stand up before handcuffing them with plastic zip ties and marching them away. The rally outside the T station began around 8:30 a.m., with the arrests following about an hour later. By 10:45 a.m. the majority of the protesters had dispersed and only a few stragglers remained.

Kate Rothstein, acting as a spokeswoman for Bend the Arc, noted that the gathering occurred about a year after the Oct. 27 mass shooting at the Tree of Life synagogue in Squirrel Hill. Then, the group had asked Mr. Trump to reject white nationalism.

"To date that has not happened," Ms. Rothstein said. "In fact, the words he continues to use doubled down."

Ms. Rothstein said that her group's act of civil disobedience was meant to prod Mr. Trump to action in denouncing white nationalism.

"He's using this false rhetoric that is endangering people's lives," Ms. Rothstein said. "It's a very public statement about how strongly we feel about this."

Ms. Rothstein was joined by her daughter, Simone, 17. The teenager talked about hate crimes increasing since Mr. Trump took office.

“You don’t have to wear a swastika on your arm to be a white nationalist or support them,” Simone said.

Among the protesters was Kevin Mosley, who identified himself as a retired state trooper.

"You don't have to just be black to be a victim of Trump's rhetoric," said Mr. Mosley, who is black.

Mr. Mosley said that when Mr. Trump ran for office, Mr. Mosley told people that a vote for him was not about economic improvement. Instead, he said he counseled people, "If you vote for him, you're putting us all at risk. This is about hate."

Mr. Mosley said that he participated Wednesday to show support for other groups, and he added that he was part of the vigils last year outside Tree of Life and the protest against Mr. Trump visiting the synagogue days after 11 worshipers were fatally shot.

"You can't just care about your own people," Mr. Mosley said.

Erika Gold Kestenberg, a diversity, equity and inclusion consultant, said the nation needs to shift its trajectory away from hate and inequity to love and justice.

“We have a president who is trying to divide our country,” she said, adding that being divisive is how people gain power. “We all need to stand in solidarity together. All people of every social identity, every race, gender, religion and immigration status.”