Taiwan issued an islandwide air raid alert after a Chinese satellite flew over its southern airspace days before a crucial presidential election.

Mobile phone users across the self-ruled island received a message warning them to "be aware for your safety".

The "presidential alert" underlined jitters ahead of pivotal presidential and legislative elections on Saturday.

China has long claimed Taiwan as part of its own territory and has been accused of interfering in the vote.

The self-governing island of 23 million people, is a key flashpoint in the tussle between China and the US for supremacy in Asia. Analysts say the elections will shape the trajectory of ties between Beijing and Washington.

The satellite was launched off the Xichang Satellite Launch Centre in China's Sichuan province at 15:03 local time, Taiwan's defence ministry said. At about the same time as the alert, China's CCTV announced that a satellite named Einstein Probe had been launched with "complete success".

Taiwan's foreign minister Joseph Wu said the launch of a satellite over Taiwanese airspace so close to the election is a "greyzone" activity, Reuters news agency reported.

"When a rocket is openly flying in our sky, some of their tubes or debris will fall in this region," said Mr Wu, who was at a news conference when the alert was sent.

"That's the reason why our national alert centre will issue this kind of alert. It has happened before."

Taiwan said its defence ministry was tracking the trajectory of the satellite to "appropriately alert and respond" to the situation. The satellite was at high altitude when it crossed Taiwan's airspace, the ministry said.

Taiwanese media say this is the first time its government has flashed an islandwide alert of this nature.

Eric Chu, the chairman of Taiwan's main opposition party Kuomintang (KMT), accused the Democratic Progressive Party's (DPP) government of fear-mongering, especially with the use of the term "missile" in the English version of the alert. It read: "[Air raid alert] Missile flyover Taiwan airspace, be aware."

"This is the first time I heard that launching a satellite can trigger such an alert. Everyone was possibly shocked, thinking it was a missile, but it turned out to be a satellite... So I think the Ministry of National Defense is trying to mislead the public by issuing such an alert," he said.

Taiwan's defence ministry later apologised for its inaccurate reference to a missile in the message to mobile phones.

According to the European Space Agency, the Einstein Probe is a partnership between the agency, the Chinese Academy of Sciences and the Max Planck Institute for Extraterrestrial Physics.

The satellite will "observe mysterious transient phenomena in the universe comparable to the flickering of fireworks, with the aim of unveiling the violent and little-known aspects of the cosmos", China's Xinhua news agency said on Tuesday.

In December, Taiwanese media had also reported on Chinese carrier rockets passing over the island. A Focus Taiwan report published on 10 December said a carrier rocket, also launched from Xichang had passed over southwestern Taiwan in outer space.