Dozens of Chinese military aircraft crossed the sensitive median line of the Taiwan Strait after China began three days of military exercises around the island.
The drills are in retaliation for a meeting between the US House of Representatives speaker Kevin McCarthy and Taiwan's president.
A Chinese amphibious landing ship - capable of transporting troops, craft and vehicles - began live-fire drills in the seas facing the Taiwan Strait on Saturday morning.
Taiwan's Defence Ministry said as of 4pm local time (9am in the UK) it spotted 71 Chinese aircraft, including fighter jets and bombers, crossing the median line that serves as an unofficial barrier between the two sides, as well as nine Chinese ships.
The People's Liberation Army said the three-day "combat readiness patrols" were a warning to those in Taiwan who want to make the island's de facto independence permanent.
The Taiwanese military said missile defence systems were activated and air and sea patrols sent to track the Chinese aircraft.
"We condemn such an irrational act that has jeopardised regional security and stability," Taiwan's defence ministry said in a statement.
China announced the drills after Taiwan's president Tsai Ing-wen returned to Taipei from Los Angeles on Thursday, where she met House Speaker Kevin McCarthy.
Taiwan split from China after a civil war in 1949 and the ruling Communist Party says the island is obliged to rejoin the mainland by force if necessary.
Beijing argues contact with foreign officials encourages Taiwanese people who want formal independence.
"This is a serious warning against the collusion and provocation between the 'Taiwan independence' separatist forces and external forces," the PLA said in a statement.
The "Joint Sword" exercises are a "necessary action to defend national sovereignty and territorial integrity".
Meanwhile, the chair of the US House Foreign Affairs Committee, Michael McCaul, has pledged to help provide training for Taiwan's armed forces and speed up the delivery of weapons during a visit to Taipei.
"We are doing everything we can in Congress to speed up these sales and get the weapons that you need to defend
yourselves," Mr McCaul said. "And we will provide training to your military - not for war, but for peace.
"Projecting weakness only invites aggression and conflict. Projecting strength provides deterrence and promotes peace."
The US has no official relations with Taiwan's government but maintains extensive informal and commercial ties. Washington is required by federal law to ensure the island of 22 million people has the means to defend itself if China attacks.
"We will never leave room for 'Taiwan independence' separatist activities in any form and will definitely take resolute measures to defeat any foreign interference," said a spokesperson for China's Taiwan Affairs Office, Zhu Fenglian, according to the official Xinhua News Agency.
"Complete reunification of our country must be realised, and it can, without doubt, be realised," Mr Zhu said.
China announced the military drills hours after French President Emmanuel Macron left the country following a meeting with President Xi Jinping and other senior leaders. He urged Beijing to bring Russia to the negotiating table over the war in Ukraine.