Hong Kong, Shenzhen and other southern Chinese megacities are bracing for super typhoon "Saola" on Friday as it threatens to become the strongest storm to hit the region in decades.

With the highest alert level of T10, Saola already forced the start of classes and stock trading in Hong Kong to be suspended while hundreds of flights were canceled regionwide.

Hong Kong's airport authority said more than 300 flights were canceled Friday, though 600 were still scheduled.

Neighboring casino hub Macau also issued its third-highest typhoon warning by mid-afternoon.

China's national weather office said Friday that Saola was 140 kilometers east-southeast of Hong Kong with sustained winds of 210 kms per hour as of 3 p.m. Friday.

Authorities warned that Saola could cause a storm surge that could lead to "serious flooding."

Neighboring Shenzhen — home to 17.7 million people — opened shelters for people to take refuge, and planned to halt public transportation.

Trains in and out of Guangdong will also be suspended from 8 p.m. to 6 p.m. Saturday.

Saola displaced thousands earlier this week as it passed the northern Philippines, but no direct casualties have been reported so far.

Southern China is frequently hit in summer and autumn by typhoons that form in the warm oceans east of the Philippines and then travel west.