Roads were jammed as residents used a five-day holiday to leave the city.
Flood waters are continuing to creep into northern districts of Bangkok but the centre remains mostly dry.
Officials have warned high tides due on Saturday, combined with the flow of run-off water from inundated central plains, could cause wider flooding.
Heavy monsoon rains have been causing flooding in Thailand since July. More than 370 people have been killed and swathes of the country affected.
Bangkok is now being hit by the flow of water south to the sea.
Authorities fear that the Chao Phraya river, which bisects Bangkok, could burst its banks when water levels rise because of unusually high seasonal tides over the weekend.
Seven districts in the north of Bangkok are flooded, with residents told they should evacuate.
On Thursday, bus stations and roads were packed as many thousands of city-dwellers headed for coastal southern and eastern resorts unaffected by the flooding.
The exodus continued on Friday, with television footage showing roads leading out of the city jammed with cars and trucks.
The government has declared a five-day holiday - Thursday until Monday - in Bangkok and 20 provinces hit by flooding to help residents tackle the situation.