"This week the president has admitted to asking the president of Ukraine to take actions which would benefit him politically," she said during remarks from the Capitol. "The actions of the Trump presidency revealed the dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security and betrayal of the integrity of our elections.
"Therefore, today I'm announcing the House of Representatives is moving forward with an official impeachment inquiry."
Pelosi said Trump "seriously violated the Constitution" by encouraging Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden, who is running for president in 2020.
She also accused acting Director of National Intelligence Joseph Maguire of unconstitutionally preventing the DNI inspector general from handing over the full complaint lodged against Trump to congressional intelligence committees.
"I can say with authority that the Trump administration's actions undermine both our national security and our intelligence and our protections of the whistle-blowers," she said.
She directed the six committees currently investigating Trump to continue doing so under the umbrella of an impeachment inquiry.
Pelosi's announcement came hours after Trump pledged to release an unredacted transcript of a phone conversation with Zelensky that a whistle-blower said was improper.
Trump said he plans to release the transcript Wednesday.
"I am currently at the United Nations representing our Country, but have authorized the release tomorrow of the complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine," he tweeted.
"You will see it was a very friendly and totally appropriate call. No pressure and, unlike [former Vice President] Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!"
Trump called Pelosi's impeachment announcement "presidential harassment" in a series of tweets after he remarks.
"Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country!" he tweeted.
"Pelosi, Nadler, Schiff and, of course, Maxine Waters! Can you believe this?
"They never even saw the transcript of the call. A total Witch Hunt!" he added.
Sources told The Washington Post last week that a whistle-blower in the intelligence community filed a complaint with the office of the director of national intelligence about a July phone conversation Trump and Zelensky had. The entire substance of the phone call hasn't been made public, but the sources said Trump made a "promise" to Zelensky.
Trump and his personal lawyer, Rudy Giuliani, have both said they have encouraged Ukraine -- in other conversations -- to carry out an investigation into Joe Biden and his son, Hunter Biden. They accused the two men of pressuring Ukraine to fire a Prosecutor General Viktor Shokin, who was investigating a company for which Hunter Biden served on the board.
Shokin's successor, Prosecutor General Yuriy Lutsenko told Bloomberg in May that the investigation into the company began before Hunter Biden joined the board, and he wasn't specifically a target of the probe.
Trump's promise to release the transcript came hours after he acknowledged he ordered White House chief of staff Mick Mulvaney to withhold tens of millions of dollars in military aid from Ukraine a week before the call with Zelensky.
Before making his address to the United Nations General Assembly in New York City, Trump told reporters he gave the order because he felt other allied nations weren't providing enough.
Congressional Democrats are calling for an impeachment investigation into the president, based on the allegation he encouraged a probe into the Bidens.
Pelosi, D-Calif., said the allegations are concerning even if Trump didn't threaten to withhold military aid.
"There is no requirement there be a quid pro quo in the conversation," she said during an appearance at The Atlantic Festival on Tuesday afternoon. If the president wants a foreign country "to investigate something -- of his political opponent -- that is self-evident that it is not right.
"We don't ask foreign governments to help us in our elections."
Biden accused Trump of "abuse of power" during remarks Tuesday afternoon.
"We have a president who believes there's no limit to his power. We have a president who believes he can do anything and get away with it. We have a president who believes he's above the law. Pursuing the leader of another nation to investigate a political opponent to help win his election is not the conduct of an American president," he said.
Biden called on the Trump administration to provide Congress with a copy of the whistle-blower's formal complaint so that lawmakers can formally investigate the allegations.
"If he continues to obstruct Congress and flaunt the law, Donald Trump will leave Congress, in my view, no choice but to initiate impeachment. That would be a tragedy, but a tragedy of his own making."
Trump called a possible impeachment inquiry a "witch hunt."
"[Democrats] have no idea how they stop me. The only way they can try is through impeachment.
"As far as withholding funds, those funds were paid," he added. "They were fully paid.
"I want other countries to put up money. I think it's unfair that we put up the money," he added. "Then other people call me. They said, 'Oh, let it go.' And I let it go. But we paid the money, the money was paid."
House Republican leader Kevin McCarthy of California criticized Pelosi's announcement in a short statement before reporters at the Capitol.
"Speaker Pelosi happens to be the speaker of this House, but she does not speak for America when it comes to this issue. She cannot decide unilaterally what happens here," he said.
"The election is over. I realize 2016 did not turn out the way Speaker Pelosi wanted it to happen, but she cannot change the laws of this Congress. It's time to put the public before politics."
Earlier Tuesday, Sen. Robert Menendez, the top Democrat on the Senate foreign relations committee, asked State Department inspector general Steve Linick to open an investigation into the decision to delay $141 million in military aid to Ukraine. The Washington Post reported the amount of the withheld aid was $400 million. Officials said the funds were released Sept. 11.
"If the president withheld the funds to extract commitments by the Ukrainian government to take action against a political opponent of the president, it would constitute an unacceptable abuse of power and subversion of U.S. foreign policy and foreign assistance funds under the purview of the State Department for the personal use of the president," Menendez wrote in a letter to Linick.
The New Jersey senator asked for answers to several questions -- including why Mulvaney asked for the delay, whether relevant laws were followed and whether any type of a review was done. He also asked for all communications between the White House and State Department and the budget office.
Menendez also sent a letter to Secretary of State Mike Pompeo to see when he first learned of the delay.
Trump acknowledged Sunday he discussed Biden on the phone call and later said he never threatened to withhold aid -- which conflicts with what he told reporters in New York on Tuesday.
"I'd withhold again," he said. "And I'll continue to withhold until such time as Europe and other nations contribute to Ukraine, because they're not doing it."
Democrats want to know whether Trump used Congress-approved approved military aid as political leverage.
Rep. Adam Schiff, chairman of the House intelligence committee, said the whistle-blower wishes to testify before the panel.
"We have been informed by the whistle-blower's counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI as to how to do so," he tweeted. "We're in touch with counsel and look forward to the whistle-blower's testimony as soon as this week."