Ms Pelosi left on Wednesday after a brief but controversial visit to Taiwan, which China regards as a breakaway province.
In response, China announced five days of “necessary and just” military drills, which will begin on Thursday.
Taiwan said 27 Chinese warplanes had already entered its air defence zone.
On Wednesday, Taiwan’s defence ministry said it had scrambled jets to warn them off.
China has said the exercises will take place in some of the world’s busiest waterways and will include “long-range live ammunition shooting”.
President Tsai Ing-wen said the country was facing “deliberately heightened military threats”.
In an effort to calm the situation, the foreign ministers from the G7 nations – Canada, France, Germany, Italy, Japan, the UK and US – released a joint statement saying China’s escalation risked destabilising the region.
“There is no justification to use a visit as a pretext for aggressive military activity in the Taiwan Strait. It is normal and routine for legislators from our countries to travel internationally,” the statement said
Ms Pelosi, the most senior US politician to visit Taiwan in 25 years, made the trip as part of a wider Asian tour. China had warned her not to travel to the island.
Accusing the US of “violating China’s sovereignty under the guise of so-called democracy”, Chinese Foreign Minister Wang Yi said: “Those who play with fire will not come to a good end and those who offend China will be punished.”
In a statement following the visit, Ms Pelosi said China cannot “prevent world leaders or anyone from travelling to Taiwan to pay respect to its flourishing democracy, to highlight its many successes and to reaffirm our commitment to continued collaboration”.
The senior US Democrat’s visit was not approved by her party colleague, President Joe Biden, who had said the American military felt it was “not a good idea right now” amid heightened tensions between the two countries.
The US walks a diplomatic tightrope with its Taiwan policy.
On the one hand, it abides by the “One China” policy, which recognises only one Chinese government, giving it formal ties with Beijing and not Taiwan.
On the other, it maintains a “robust unofficial” relationship with the island, which includes selling weapons for Taiwan to defend itself.
China and Taiwan: The basics
- Why do China and Taiwan have poor relations? China and Taiwan were divided during a civil war in the 1940s, but Beijing insists the island will be reclaimed at some point, by force if necessary
- How is Taiwan governed? The island has its own constitution, democratically elected leaders, and about 300,000 active troops in its armed forces
- Who recognises Taiwan? Only a few countries recognise Taiwan. Most recognise the Chinese government in Beijing instead. The US has no official ties with Taiwan but does have a law which requires it to provide the island with the means to defend itself.