Updated at 4:30 p.m. ET

The former U.S. ambassador to Ukraine told Congress on Friday she was recalled after a smear campaign led by President Trump's allies — and Trump criticized her on Twitter even as she testified live on television.

Ambassador Marie Yovanovitch appeared at Democrats' second open impeachment hearing to discuss her career and the circumstances under which her posting to Kyiv was prematurely halted earlier this year.

Trump posted two tweets about Yovanovitch during the session that linked her to problems in the troubled countries in which she had been posted and restated the president's power to appoint and remove diplomats as he wishes.

Intelligence Committee Chairman Adam Schiff, D-Calif., paused the testimony to read Yovanovitch portions of the tweets during the hearing. He asked what effect she thought it might have on witnesses' willingness to come forward and expose wrongdoing.

"It's very intimidating," she said, although Yovanovitch said she couldn't address what Trump's intentions might have been.

Responded Schiff: "Well, I want to let you know, Ambassador, that some of us here take witness intimidation very, very seriously."

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The chairman repeated his warning to reporters during a break in the hearing. Afterward, Schiff linked what he called the intimidation with an "incriminating" pattern of conduct by Trump.

After an unrelated event at the White House on health care prices, Trump told reporters that he had been exercising his "freedom of speech" and did not feel that he had been intimidating.

"I have the right to speak," Trump said. "I have freedom of speech just as other people do. But they've taken away the Republicans' rights."

Witness about early phase of saga

Yovanovitch was a key witness about the earliest phase of the Ukraine affair.

She lost her post in Ukraine's capital, Kyiv, earlier this year after what has been described as a smear campaign by Trump's attorney, Rudy Giuliani, and associates Lev Parnas and Igor Fruman, witnesses have told House investigators.

Those three worked with Ukraine's then-Prosecutor General Yuri Lutsenko to spread what Yovanovitch called lies about her, including that she was disloyal to Trump and that she had given Lutsenko a list of Ukrainians she didn't want prosecuted.

Parnas and Fruman have been arrested and charged with violating U.S. campaign finance law; they have pleaded not guilty. An attorney for Parnas has told NPR he's open to working with congressional impeachment investigators, but the outlines of that aren't clear.

Prosecutors may be investigating whether Giuliani stood to profit from a natural gas business linked with Parnas and Fruman, The Wall Street Journal reported on Friday.

Meanwhile at the Capitol, Schiff said that Yovanovitch's ouster was a necessary prologue to the main story about Ukraine — that Trump and Giuliani had to push her out of the way to put in place the shadow lane for communication they wanted outside of officialdom.

"Getting rid of Ambassador Yovanovitch helped set the stage for an irregular channel that could pursue the two investigations that mattered so much to the president: the 2016 conspiracy theory, and most important, an investigation into the 2020 political opponent he apparently feared most, Joe Biden," Schiff said.

Military assistance for Ukraine was frozen by the White House for a period of weeks this summer, then released. Ukraine's president, however, never made the public commitment for investigations that intermediaries have said they sought on Trump's behalf.

Risk seen to U.S. diplomacy

Yovanovitch warned Congress on Friday that the precedent set by her recall from Ukraine weakened Washington's ability to work with developing nations around the world.

The U.S. has set an example for countries seeking to build institutions and fight corruption, Yovanovitch said — but her ouster showed that enemies in Ukraine could exploit American officials to get rid of someone they disliked.

"How could our system fail like this? How is it that foreign corrupt interests could manipulate our government?" she asked.

Yovanovitch acknowledged that Trump has the power to appoint and remove diplomats as he pleases, but she argued that if an American representative can be "kneecapped" for political reasons, it dilutes U.S. power.

"Shady interests the world over have learned how little it takes to remove an American ambassador who does not give them what they want," Yovanovitch said.