It accused Ms Pelosi, the most senior US politician in 25 years to visit the island China claims as its own, of “playing with fire”.
“Those who play with fire will perish by it,” Beijing warned in a statement.
Ms Pelosi said her visit honoured “America’s unwavering commitment to supporting Taiwan’s vibrant democracy” and did not contradict US policy.
As her plane touched down, Chinese state media reported that its military jets were crossing the Taiwan strait. Taiwan denied those reports at the time – but later said that more than 20 Chinese military planes had entered its air defence zone on Tuesday.
China – which sees Taiwan as a breakaway province which will one day reunite with it – has previously warned that its armed forces “will not stand idly by”.
Within an hour of the plane’s arrival, it announced that the People’s Liberation Army will conduct a series of live-fire military drills in the air and at sea around Taiwan later this week – warning ships and aircraft not to enter the affected areas.
It follows days of escalating tensions ahead of the visit, in which Chinese warplanes had already ventured out as far as the median line, the unofficial dividing line separating China and Taiwan in the waters between them.
In her statement, Ms Pelosi said: “America’s solidarity with the 23 million people of Taiwan is more important today than ever, as the world faces a choice between autocracy and democracy.”
And in an article published in the Washington Post newspaper at the same time, Ms Pelosi also wrote that Taiwan’s “robust democracy… is under threat”.
“In the face of the Chinese Communist Party’s (CCP) accelerating aggression, our congressional delegation’s visit should be seen as an unequivocal statement that America stands with Taiwan, our democratic partner, as it defends itself and its freedom,” she said.
After her arrival, White House national security spokesperson John Kirby told reporters that the trip was consistent with the US’s long-standing policy towards China and did not violate the country’s sovereignty.
“There’s no reason for this visit to become a spurring event for a crisis or conflict,” he said.
While Ms Pelosi’s visit had been the subject of huge international speculation for days, it had been shrouded in secrecy until the last minute.
When she set off on a tour of Asia on Sunday, there was no mention of Taiwan on her official itinerary, which included Singapore, Malaysia, South Korea and Japan.
The White House has been open in its opposition to any such trip, and President Joe Biden said the military assessed it as “not a good idea”.
But after Ms Pelosi landed, the White House’s Kirby told CNN this visit was similar to previous trips by other officials.
“There is no reason for this to erupt into conflict. There’s no change to our policy. This is absolutely consistent with it.”
Addressing the strong reaction from China, he said: “The United States is not going to be intimidated by threats.”
There are signs that Ms Pelosi’s visit is garnering bipartisan support from Republican opponents who would usually be very unlikely to count themselves among her fan base, says the BBC’s John Sudworth in Washington.
The latest opposition politician to back her, Republican Senator Dan Sullivan, said: “You do not want the Chinese Communist party dictating to senior American leaders where they can and cannot travel.”
He added: “She’s there, we’re supporting the trip and we’re closing ranks behind her.”
China exerts international pressure on other nations to accept its “One China” principle – that there is only one Chinese nation, based in Beijing. Only 15 nations in the world have formal diplomatic relations with Taiwan.
Long-standing US policy has been to recognise the Beijing government, but also maintain “robust unofficial” relations with Taiwan. That includes selling weapons for Taiwan to defend itself.
As Speaker of the US House of Representatives, Ms Pelosi is second in line for the US presidency after the vice-president. Yet she is also a long-standing critic of Beijing.
As a congresswoman in 1991, two years after the Chinese government cracked down hard on protesters in Tiananmen Square, she visited the site of the demonstrations and unfurled a banner in memory of those who died – sparking an angry response from the government there.
Ms Pelosi is expected to stay overnight, Taiwanese media have reported, and meet members of the Legislative Yuan on Wednesday, as well as Taiwan’s President Tsai Ing-Wen.
