TAITUNG, Taiwan, Oct 5 (Reuters) - Typhoon Koinu began brushing past the rural far southern tip of Taiwan on Thursday, bringing lashing rains and strong winds as a swathe of cities across the island cancelled work and classes.

Koinu made landfall on Taiwan's Hengchun peninsula as a category four typhoon - indicating winds of up to 252kph (156mph) - but is forecast to weaken as it crosses into the Taiwan Strait and heads toward the Chinese province of Guangdong, according to Tropical Storm Risk.

The heaviest rain is falling along mountainous and sparsely populated parts of Pingtung county in the south, and the east coast counties of Taitung and Hualien, but the typhoon will also affect the major southern port city of Kaohsiung.

Most cities and counties in Taiwan declared a day off work and school on Thursday, though the capital Taipei, home to the stock and financial markets, is not affected and operating as normal.

Taiwan's two main domestic airlines, UNI Air and Mandarin Airlines, cancelled most of their flights for Thursday, while ferries to outlying islands were also stopped. (Reporting by Fabian Hamacher and Carlos Garcia; Writing by Ben Blanchard. Editing by Gerry Doyle)