Three Chinese warships sailed in waters surrounding the self-ruled island, while a fighter jet and an anti-submarine helicopter also crossed the island's air defence identification zone (ADIZ), Taiwan's defence ministry said.
Hours before Tsai met McCarthy in Los Angeles on her way back from Latin America, China's Shandong aircraft carrier sailed through Taiwan's southeastern waters on its way to the western Pacific.
Tsai told reporters her government was committed to ensuring "the free and democratic way of life of the people of Taiwan" before she left Los Angeles where she was stopping on her way back from Latin America.
"We also hope to do our best to maintain peace and stability between the two sides," she added.
Beijing said on Friday that "Taiwan is an inseparable part of China", after repeatedly warning against the meeting.
"China's sovereignty and territorial integrity will never be divided," foreign ministry spokeswoman Mao Ning said at a regular press briefing.
"The future of Taiwan lies in reunification with the motherland."
AFP journalists on Pingtan island, China's nearest outpost to Taiwan, saw a military ship and at least three army helicopters transiting through the strait on Friday afternoon.
The silver-grey Dongtuo-859 tugboat cruised southwards, about one kilometre from the shore, while the two aircraft flew northwards over the waters at fairly low altitudes.
It was not immediately clear if the movements represented an enhancement of normal patrols that Beijing conducts in the region.
Last August, China deployed warships, missiles and fighter jets around Taiwan for its largest show of force in years, following a trip to the island by McCarthy's predecessor Nancy Pelosi.
Beijing's response to the Tsai-McCarthy meeting has so far been more muted and China's ADIZ incursions in the past two days have not risen beyond the activities of an average week.
But Taiwan is still on alert, with Premier Chen Chien-jen warning on Friday that Taipei's defence and security agencies were keeping a close eye on developments.
On Thursday, Taiwan's defence ministry said three warships had been detected around the Taiwan Strait and one Chinese naval helicopter crossed the island's ADIZ.
The display prompted calls from the United States asking China "to cease its military, diplomatic and economic pressure against Taiwan and instead engage in meaningful diplomacy".
The ADIZ is not the same as Taiwan's territorial airspace, including a far greater area that overlaps with part of China's own ADIZ and even some of the mainland.
McCarthy, who is second in line for the US presidency, had originally planned to go to Taiwan himself but opted instead to meet Tsai in California.
The decision was viewed as a compromise that would underscore support for Taiwan but avoid inflaming tensions with China, a move analysts say has so far proven successful.
Tsai said on Thursday that it was "quite common for us to meet our US friends during transits".
"I also hope the Chinese side can exercise self-restraint and don't overreact," she added.
McCarthy had vowed that US arms sales to Taiwan -- which have infuriated the Chinese leadership -- would continue, in what he said was a proven strategy to dissuade aggression.
"It is a critical lesson that we learned through Ukraine, that the idea of just sanctions in the future is not going to stop somebody" who wants to wage war, he said.
On Friday, China slapped sanctions on Taipei's de facto ambassador to the United States, Hsiao Bi-khim, banning her from entering China and accusing her of "deliberately inciting cross-strait confrontation".
Taiwan condemned the sanctions, saying Beijing was attempting to "further suppress our country's international space".
"Coercion and suppression will not change the objective facts, but will only strengthen our government's belief in upholding freedom and democracy," Taiwan's foreign office said in a statement.
Beijing also announced sanctions against the Hudson Institute, a Washington-based conservative think tank, as well as the Ronald Reagan Presidential Library, which hosted the meeting.
The two organisations are now barred from engaging in transactions and cooperation with Chinese entities.
On Thursday, Taiwan said Chinese coast guard vessels were "obstructing" trade by carrying out on-site inspections on cargo and passenger ships.
Tsai said Taipei's national security team was "closely monitoring the situation" to ensure the safety of its ships "and to prevent China's interference in our territorial waters".