A man, aged 35 to 40, jumped off the Bhumibol 2 bridge early on Monday despite of an attempt by a bridge security guard to stop him.
He plunged 50 metres from the bridge into the Chao Phraya river and went missing under the cover of darkness beneath.
According to the security guards at the bridge, a 170-centimetre tall man in long white T-shirt and dark colour trouser was first seen in the bridge's video cameras climbing over the rail fence of the Poo Chao Saming Prai road toward the Bhumibol 2 bridge.
They immediately alerted a security guard on the bridge to stop the man from walking into the bridge.
The security guard on the bridge identified as Sompong Inhun, 57, went out to check and saw a man walking up the bridge to the middle.
He then told the man that the bridge area is off-limit to pedestrians but he didn't listen.
Instead, he climbed over the bridge railing and jumped off to the river 50 metres below.
Sompong immediatly alerted the control centre which later called in help from Phra Pradaeng police and rescuers from Por Teck Tueng charity foundation.
But rescue operation under the river could not start as divers reported the strong current in the middle of the river, as well as darkness.
They decided to resume the search in the morning.
Police also asked relatives whose family member with initial identity matching the man to report the police at both Phrase Pradaeng and Bang Kholaem police stations for information.
Source: Thai Public Broadcasting Service (ThaiPBS)