Former President Donald Trump has been making calls to Congressional allies on Capitol Hill Thursday night, urging them to go on the offensive and defend him following the news of the indictment by a Manhattan grand jury, sources familiar with the matter tell ABC News.

Trump has been asking members who support him to firm up their support and rally behind him, the sources said.

-ABC News' Rachel Scott, Will Steakin and Katherine Faulders

Some of Trump's confirmed or prospective rivals for the 2024 Republican presidential nomination were among those who spoke out Thursday night in the wake of the news of the former president's indictment.

In an interview with CNN on Thursday night, former Vice President Mike Pence called it an "outrage," arguing that the case against Trump is "tenuous" and will "only further serve to divide our country."

Florida Gov. Ron DeSantis, whom sources have told ABC News is expected to launch a presidential campaign in the coming months, tweeted that the indictment was "un-American" and "a weaponization of the legal system," adding that Florida would "not assist in an extradition request."

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-ABC News' Alexandra Hutzler and Mariam Khan

California Rep. Adam Schiff said "it's a sober moment for the country," telling Linsey Davis on ABC News Live on Thursday that the dignity of the Oval Office "will be tarnished by the conduct of the former president by his being charged criminally."

"I think you have to be guided by the facts and the law, and you have to set aside the political calendar and do what the law requires," said Schiff, a leading Democrat in the House. "I think that's the obligation of a district attorney, and I think that was done here. How this cuts politically, I really don't know. That, to me, is very secondary."

-ABC News' Imtiyaz Delawala and Anna Katharine Ping

Demonstrators gathered outside Manhattan Criminal Court and near Donald Trump's Mar-a-Lago estate in Florida in the wake of the indictment Thursday evening.

In Manhattan, a giant sign that stated "Trump lies all the time" could be seen unfurled outside Manhattan Criminal Court, where police had erected barricades last week ahead of a possible indictment.

Meanwhile, several supporters gathered near Mar-a-Lago with Trump 2024 flags and signs.

A white powder was discovered in the mailroom at 80 Centre Street, where the Manhattan District Attorney has offices and where a grand jury has been meeting to hear evidence in former President Donald Trump's case, according to a court official. The powder was determined to be non-hazardous, officials said.

The powder came in an envelope addressed to "Alvin," an apparent reference to Manhattan District Attorney Alvin Bragg, according police sources.

Inside the envelope was a letter containing the typewritten message, “Alvin: I am going to kill you," with 13 exclamation points, according to sources.

This envelope followed a series of unfounded threats that targeted municipal offices in New York this week.

"For three days we got four emails," Susan Stetzer, district manager at Manhattan Community Board 3, told ABC News on Friday.

At least one of the messages prompted the court to pause a hearing in the New York Attorney General’s civil lawsuit against Trump.

None of the email messages mentioned Trump by name. One included what Stetzer described as a "horrible homophobic rant."

According to Stetzer, the messages came from @mail.ru domains and some contained Cyrillic characters. The FBI is aware but does not immediately assess that the emails came from Russia, according to a law enforcement official.

"We did not get one today so I’m hoping it stops," Stetzer said.

New York City courthouses will see increased security, the Office of Court Administration said Friday.