BEIJING, June 9 (Reuters) - Non-stop heavy rains lashed parts of southwest China on Friday, triggering floods in cities, engulfing roads and partially submerging buildings.

A particularly harsh first bout of summer rains known locally as "dragon boat water" saw the city of Beihai in Guangxi log 453 millimetres on Thursday. That was a regional daily record for June, according to the China Meteorological Administration.

Cars were half underwater in flooded Beihai streets and at one multi-storey building, water cascaded down a staircase as firefighters raced to rescue its residents, videos circulating on social media showed.

The nearby city of Yulin had had 35 hours of rain as of 7 a.m. local time on Friday (2300 GMT Thursday), broadcaster CCTV reported.

Rain is forecast to continue in southern China over the coming days while the northeast is also expected to be hit by sudden thunderstorms, the weather bureau reported.

China, prone to floods, is increasingly warning of more extreme weather due to climate change. Guangxi experienced a rare extreme drought in May, with rainfall plunging to 60-year lows.

The central province of Henan, the granary of China, was recently struck by heavy rainfall that caused crops to sprout or be hit by blight, triggering concerns about food security.