Taipei, Taiwan - Millions of people were forced indoors and Taiwan's mobile internet speeds were slowed for 30 minutes on Thursday during a simulation of a Chinese attack on the island democracy.

The drill was part of Taiwan's annual "Han Kuang" military exercises that aim to prepare the island of 23 million people for a potential Chinese invasion.

Loud sirens sounded across the capital Taipei at 2:30PM local time, sending people into underground carparks, subway stations, shopping malls, and office buildings in the annual air raid drill.

Normally busy streets became eerily quiet as police directed traffic to pull over and waved people into shelters.

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This year's drill also included a reduction in mobile internet speeds for the first time, briefly stifling access to video calls, live streams, and food delivery apps during hypothetical Chinese air attacks on communications infrastructure.

"Whether it's going to reflect a total internet shutdown, I think is unlikely. But I think having people get a sense of how it may feel to have an internet disruption is important," Ben Lewis of the analysis website PLATracker told the AFP.

"I think also to test the resilience of civilian communications and internet systems is very important," he said.

During the 10-day Han Kuang drills that end Friday, Taiwan's military staged combat scenarios for its troops and reservists to practice responding to a Chinese attack.

An exercise in the underground car park of a hospital near Taipei on Thursday demonstrated a war-time medical facility.

The nationwide drill comes alongside numerous military exercises designed to simulate an invasion by China.

Beijing has repeatedly threatened to annex the self-ruled island, and operate regular live-far exercises in waters not far off its coastline.