U.S. House of Representatives Speaker Nancy Pelosi left Taiwan on Wednesday after pledging American solidarity and hailing its democracy, while China furiously condemned her visit to the self-ruled island that Beijing claims as its own.

China demonstrated its outrage over the highest-level U.S. visit to the island in 25 years with a burst of military activity in surrounding waters, summoning the U.S. ambassador in Beijing and halting several agricultural imports from Taiwan.

Some of China's planned military exercises were to take place within Taiwan's 12 nautical mile sea and air territory, according to Taiwan's Defense Ministry, an unprecedented move a senior defense official described to reporters as "amounting to a sea and air blockade of Taiwan."

Pelosi arrived with a congressional delegation on her unannounced but closely watched visit late on Tuesday, defying China's repeated warnings, in what she said shows unwavering U.S. commitment to Taiwan's democracy.

"Today the world faces a choice between democracy and autocracy," she said in a short speech during a meeting with Taiwanese President Tsai Ing-wen. "America’s determination to preserve democracy, here in Taiwan and around the world, remains ironclad."

"Our delegation came to Taiwan to make unequivocally clear that we will not abandon Taiwan," Pelosi told Tsai, who Beijing suspects of pushing for formal independence – a red line for China.

"Now, more than ever, America's solidarity with Taiwan is crucial, and that's the message we are bringing here, today," she said during her roughly 19-hour visit.

A long-time China critic, especially on human rights, Pelosi met with a former Tiananmen activist, a Hong Kong bookseller who had been detained by China and a Taiwanese activist recently released by China.

Fury on the mainland over the 82-year-old Democrat's defiance of Beijing was evident all over Chinese social media, with one blogger railing: "this old she-devil, she actually dares to come!"

The last U.S. house speaker to go to Taiwan was Newt Gingrich, in 1997. But Pelosi's visit comes amid sharply deteriorating Sino-U.S. relations, and China has emerged as a far more powerful economic, military and geopolitical force during the past quarter century.

China considers Taiwan part of its territory and has never renounced using force to bring it under its control. The U.S. warned China against using the visit as a pretext for military action against Taiwan.

In retaliation, China's customs department announced a suspension of imports of citrus fruits, chilled white striped hairtail and frozen horse mackerel from Taiwan, while its Commerce Ministry banned the export of natural sand to Taiwan.

Military drills

Pelosi's visit, which has been blasted in official Chinese news outlets, was the dominant topic on China's heavily censored social media, with many users urging Beijing to invade the island in retaliation and expressing dismay that military action had not been taken to block her arrival. A live-tracker of her plane on China's WeChat was watched by 22 million.

China's Twitter-like Weibo platform crashed just before Pelosi's landing, which Weibo blamed on overstretched broadband capacity, without mentioning Taiwan.

Shortly after Pelosi's arrival, China's military announced joint air and sea drills near Taiwan and test launches of conventional missiles in the sea east of the island, with Chinese state news agency Xinhua describing live-fire drills and other exercises around Taiwan from Thursday to Sunday.

China's Foreign Ministry said Pelosi's visit seriously damages peace and stability in the Taiwan Strait, "has a severe impact on the political foundation of China-U.S. relations, and seriously infringes upon China's sovereignty and territorial integrity."

Before Pelosi's arrival, Chinese warplanes buzzed the line dividing the Taiwan Strait. The Chinese military said it was on high alert and will launch "targeted military operations" in response to Pelosi's visit.

White House National Security Council spokesperson John Kirby said Tuesday after Pelosi's arrival that the United States "is not going to be intimidated" by China's threats or bellicose rhetoric and that there is no reason her visit should precipitate a crisis or conflict.

Kirby said China might engage in "economic coercion" toward Taiwan, adding that the impact on U.S.-China relations will depend on Beijing's actions in coming days and weeks.

'China's ambition'

The United States has no official diplomatic relations with Taiwan but is bound by American law to provide it with the means to defend itself. China views visits by U.S. officials to Taiwan as sending an encouraging signal to the pro-independence camp on the island. Taiwan rejects China's sovereignty claims, saying only the Taiwanese people can decide the island's future.

Taiwan's cabinet on Wednesday said the military has increased its alertness level. The island's defense ministry said 21 Chinese aircraft entered its air defense identification zone on Tuesday, and that China was attempting to threaten key ports and cities with drills in the surrounding waters.

"The so-called drill areas are falling within the busiest international channels in the Indo-Pacific region," a senior Taiwan official familiar with its security planning told Reuters on Wednesday.

"We can see China's ambition: to make the Taiwan Strait non-international waters, as well as making the entire area west of the first island chain in the western pacific its sphere of influence," the person said.