"It gravely undermines peace and stability across the Taiwan Strait, and sends a seriously wrong signal to the separatist forces for 'Taiwan independence,' " China's Ministry of Foreign Affairs said in a statement. "China firmly opposes and sternly condemns this, and has made serious démarche and strong protest to the United States."

China's military said it was on "high alert" and would conduct exercises around Taiwan in response to Pelosi's trip, saying in statements it was launching a series "targeted military operations to counteract the situation."

Taiwan's Ministry of National Defense said that 21 Chinese warplanes made incursions into its air defense identification zone (ADIZ) on Tuesday. In response, the Taiwanese military issued radio warnings and deployed air defense missile systems to monitor the activities, it added.

China frequently sends warplanes into Taiwan's self-declared ADIZ. The highest number of incursions ever recorded was on October 4 last year, when 56 military planes flew into the area on the same day. An ADIZ is unilaterally imposed and distinct from sovereign airspace, which is defined under international law as extending 12 nautical miles from a territory's shoreline.

US ambassador summoned

China's anger was underlined when Chinese Deputy Foreign Minister Xie Feng summoned the US Ambassador to China, Nicholas Burns, late on Tuesday evening local time to protest the visit, Chinese state media outlet CCTV reported on Wednesday.

Xie accused Pelosi of "deliberately provoking and playing with fire against people's will," saying her visit is "extremely egregious" and the consequences will be "extremely serious."

"Anyone who attempts to manipulate the Taiwan issue to gain political gain ... will eventually be nailed to the pillar of shame in history," Xie said, according to the state media report.

Taiwan's foreign ministry welcomed Pelosi on Tuesday evening. "It is believed that the visit of Speaker Pelosi and other heavyweight members of Congress will strengthen the close and friendly relationship between Taiwan and the United States and further deepen cooperation between the two sides in various fields globally," the ministry said in a statement.

Taipei Mayor Ko Wen-je also embraced Pelosi's visit but cautioned that the self-governing island should avoid being put "between a rock and hard place" in US-China relations.

"Taiwan should keep its own agency. We are friends with the US and Japan, and we don't have to be on bad terms with China," Ko said in a statement. "Taiwan is in between the US and China. We should keep an open communication channel with China and the US to effectively avoid crises."

Pelosi to meet with Taiwan's president Wednesday

The House speaker is expected to visit Taiwan's presidential office and parliament on Wednesday morning (local time), a senior Taiwanese official told CNN. She will first visit the parliament before heading to the presidential office for a meeting with Taiwan's President Tsai Ing-wen, the official said.

Pelosi is expected to depart Taiwan later on Wednesday, according to a news release issued by the foreign ministry. The official was not authorized to speak about Pelosi's travel plans that have not been publicized.

Pelosi is traveling with House Foreign Affairs Chairman Gregory Meeks of New York, Veterans' Affairs Chairman Mark Takano of California and Reps. Suzan DelBene of Washington state, Raja Krishnamoorthi of Illinois and Andy Kim of New Jersey.

The American Institute in Taiwan said Pelosi's delegation will meet with senior Taiwanese leaders "to discuss US-Taiwan relations, peace and security, economic growth and trade, the Covid-19 pandemic, the climate crisis, human rights, democratic governance, and other significant issues of mutual interest."

Pelosi's stop in Taiwan was not listed on the itinerary of her congressional visit to Asia, but the stop had been discussed for weeks in the lead up to her trip. The potential stop prompted warnings from China as well as the Biden administration, which has briefed the speaker about the risks of visiting the democratic, self-governing island, which China claims as part of its territory.

The White House said Tuesday that Pelosi's trip was consistent with US policy on Taiwan, and that the US would be watching China's actions closely after Pelosi leaves.

"Obviously, we're going to be watching this closely. There is no reason for this visit to become a spurring event for a crisis or a conflict or for a pretext that the Chinese might try to whip up for some sort of military action," National Security Council coordinator for strategic communications John Kirby said on CNN's "At this Hour with Kate Bolduan" on Tuesday.

"Of course, we're concerned about that, which is why part and parcel of her trip is to reaffirm the United States commitment under the Taiwan Relations Act to help Taiwan with its self-defense," Kirby added. "Again, there's no reason for this to erupt in to conflict. There's no change to our policy. This is absolutely consistent with it. And we're just going to watch as things unfold."

Bipartisan praise

A group of more than two dozen Senate Republicans, including Senate GOP Leader Mitch McConnell, issued a statement supporting Pelosi's congressional delegation, which was all Democrats, landing in Taiwan.

"We support Speaker of the House of Representatives Nancy Pelosi's trip to Taiwan," the Republicans said. "For decades, members of the United States Congress, including previous Speakers of the House, have travelled to Taiwan. This travel is consistent with the United States' One China policy to which we are committed. We are also committed now, more than ever, to all elements of the Taiwan Relations Act."

McConnell praised Pelosi's arrival in Taiwan, saying Tuesday that he believes "she has every right to go."

"It's been unseemly and counterproductive for President Biden and his aides to have publicly sought to deter her from doing so," the Kentucky Republican said. "I welcome the speaker's display of support for Taiwan's democracy, but I hope she returns from Asia more mindful of the military dimensions of the Chinese threat, and more committed to working with Republican to address the changing balance of military power out in the region."

US President Joe Biden said publicly before Pelosi's travel that the US military did not believe it was a good time for Pelosi to visit Taiwan, but he stopped short of telling her directly not to go, two sources previously told CNN.

Biden did not speak to Pelosi before her trip, Kirby said Tuesday.

The issue of Taiwan remains among the most contentious in US-China relations. Biden and his Chinese counterpart, Xi Jinping, discussed it at length during a phone call last week that lasted more than two hours.

Administration officials are concerned that Pelosi's trip comes at a particularly tense moment, as Xi is expected to seek an unprecedented third term at the upcoming Chinese Communist Party congress. Chinese party officials are expected to begin laying the groundwork for that conference in the coming weeks, putting pressure on the leadership in Beijing to show strength.

While Biden has not endorsed Pelosi's visit, US officials believe Chinese leadership may be conflating the House speaker's trip with an official administration visit, and they're concerned that China doesn't separate Pelosi from Biden, much, if at all, since both are Democrats.

Pelosi has long been a China hawk in Congress. She's previously met with pro-democracy dissidents and the Dalai Lama -- the exiled Tibetan spiritual leader who remains a thorn in the side of the Chinese government. She also helped display a black-and-white banner in Beijing's Tiananmen Square two years after the 1989 massacre, and in recent years she's voiced support for the pro-democracy protests in Hong Kong.

This story has been updated with additional developments.