The operator of Bangkok's Skytrain services has come out to defend its safety procedures, after it came under heavy criticism following an assault on a woman at Kheha station on Tuesday morning.

Sumit Srisantum of Bangkok Mass Transit System Plc (BTSC), which operates the BTS Green Line, said security guards responded immediately upon being told by a passer-by that a man had exposed himself on the escalator.

He rejected criticism that BTS staff were slow to respond, saying the man was first spotted on the escalator at about 9.38am, and the assault took place just 33 seconds after he was sighted.

Security guards reached the scene at 9.39am, or within a minute of the incident happening, and immediately chased the man, but lost sight of him.

At 9.40am the security guard could be seen returning to the scene of the assault to aid the victim and make a police report.

The man was arrested just hours later.

Mr Sumit said the quick arrest was possible due to the actions of the security guard and CCTV cameras at Skytrain stations, of which there are at least 4,500 across 60 stations on the Green and Gold lines.

The 26-year-old suspect later admitted to police to being high on drugs.

The victim, who worked just a few stations from where she was assaulted, said she cried out for help but no one came to her rescue until another passenger arrived on the top of the escalator, which scared the attacker away.

The victim said that she is now afraid of going outside alone, adding that she has completely lost trust in the city's Skytrain system.