The Hong Kong Observatory issued the T8 Tropical Cyclone Warning Signal at 6.40pm today.
The government advises members of the public with long or difficult journeys home or having to return to outlying islands to begin their journey now.
“The water level may rise and cause flooding in the evening,” said Sandy Song Man-kuen, the observatory’s senior scientific officer.
The Macau Observatory said the No 8 northwest storm signal would be hoisted before 7pm.
The Hong Kong Obsevatory said that Usagi would make landfall to the east of Hong Kong and will skirt the territory at around 100km or less to the north later tonight and early tomorrow morning.
A storm surge induced by Usagi may still lead to flooding in low-lying areas tomorrow morning. The Observatory said the public should be on the alert and take precautions against strong winds and flooding as early as possible.
China's National Meteorological Centre has said that Usagi is likely to land tonight somewhere between Huilai and Huidong on Guangdong's eastern coast, only 50km east of Hong Kong.
The Airport Authority said 166 arriving and 187 departing flights had been cancelled so far, while 42 arrivals and 25 departures had been delayed. Flight operations may be affected today and tomorrow, and passengers are advised to contact their airlines for the latest flight information.
Hundreds of travellers are stranded in the airport in Chep Lap Kok waiting for rearrangements of their flights .
— AFP Photo Department (@AFPphoto) September 22, 2013 
Other transport services have also been affected. The New World First Ferry said the last inter-island services would depart from 2.50pm onwards, but added that it may have to suspend services before the typhoon signal No 8 was hoisted.
The Star Ferry said services connecting Central and Wanchai to Tsim Sha Tsui would be suspended within one hour after the signal No 8 was hoisted.
Typhoon Usagi creating 42ft waves according to the Pacific Disaster Center
— Alex Millson (@MillsonAlex) September 22, 2013 
The Transport Department said ferries connecting Tuen Mun, Tung Chung, Sha Lo Wan and Tai O had been halted.
According to Cotai Jet, services going to and from Macau would be gradually suspended in the afternoon, but would be cut-off pending the signal No 8.
The Hong Kong Jockey Club cancelled today's race meeting at Sha Tin Racecourse and said it planned to reschedule for Wednesday October 30, subject to government approval.
About a dozen hardcore surfers remained in Big Wave Bay an hour or two before the No 8 signal was hoisted. They said the wave was not particularly strong. The beach was so popular that the 30 parking lots were almost full.
But some surfers did give up. "The wave is really strong. It could be more dangerous later," said Ted Wong.
Technicians at the Daya Bay nuclear power plant in Guangdong took steps to ensure the installation was secure. The Security Bureau said the government had contingency plans in case of the nuclear plant was damaged.
Observatory senior scientific officer Mok Hing-yim warned of "astronomical" high tides and a storm surge.
Usagi will be closest to Hong Kong this evening. Shoppers cleared supermarket shelves of food as word spread that it could deliver a direct hit to the city.
The storm lashed the east and south coasts of Taiwan as it passed between the island and the Philippines' Batanes island chain yesterday. While the storm had weakened from super typhoon status, at least two people were killed in the Philippines and two others were missing.
"This is the strongest typhoon to hit Batanes in 25 years," Dina Abad, congresswoman for the Batanes islands, said. "The howling winds began at midnight and they churned up to eight-metre waves that damaged the port and sank moored fishing boats."
Taiwan's defence ministry deployed more than 1,600 soldiers to evacuate people living in areas vulnerable to flooding and landslides, according to the United Daily News. Mudslide warnings were issued for seven counties. Taiwan cancelled 82 domestic and international flights yesterday, with 33 delayed.
In Hong Kong, Cathay Pacific and Dragonair cancelled all flights into and out of the city from 6pm today. China Airlines cancelled 26 flights between Hong Kong and Taiwan and Indonesia. Hong Kong Airlines and Hong Kong Express also cancelled at least six flights.
Due to the impact of Super Typhoon Usagi,operations at Hong Kong will stop from 18:00 on 22Sep through 23Sep.More at http://www.cathaypacific.com 
— Cathay Pacific (@cathaypacific) September 21, 2013 
Mok said that if the typhoon signal No 8 was issued, Usagi would be the strongest typhoon to hit Hong Kong since Typhoon Hope in 1979, which killed 12 people and injured 260.
Additional reporting by Agence France-Presse