Around 100 protesters gathered at a section of a 15km flood barrier in the district of Don Mueang on Sunday, removing small sandbags and damaging larger ones that were too heavy to move.
A local police chief said that residents created a shallow 7m wide opening in the wall in recent days to relieve badly-flooded neighbourhoods, and on Sunday they managed to deepen the gap to around a metre.
Bangkok authorities are expected to repair the damage later on Monday, but demonstrators have threatened to expand their protest if that happens.
"[Residents] said yesterday if the government fixes it, they will block the toll road," a major route linking Bangkok to the north, police colonel Rangsan Praditphol said, adding that the situation was calm on Monday morning.
The floodwall is seen as a key defence preventing run-off waters from the north from swamping downtown Bangkok, which is home to the city's luxury hotels, office buildings and shopping malls.
Residents behind the wall say they have been struggling to cope with waist-deep water for up to a month.
Thailand's worst floods in half a century, triggered by months of unusually heavy monsoon rains, have left at least 562 people dead around the kingdom and damaged millions of homes and livelihoods.
The waters have also swamped Bangkok's number two airport, Don Mueang, which mostly serves domestic destinations, forcing its closure in late October.
The kingdom's main air gateway Suvarnabhumi, east of the city centre, is operating as normal.