Rail operator SMRT was unable to fix a signalling fault that affected service on the Circle Line for the whole of yesterday.
The glitch, which the operator described as "intermittent signalling issues", slowed train speeds and reduced service frequency during the morning peak period.
Commuters took to social media to complain of packed stations. Other users tweeted that faulty sensors were causing train doors to shut on commuters.
One commuter told Channel News Asia that the crowd at Buona Vista interchange was so huge, she could only get on the fourth train.
She did not hear any announcements about the fault.
Commuter Kenneth Koh tweeted at 8.37am: "Serangoon station platform is overflowing."
Another user named Venus said Bishan station had "turned into a madhouse", posting a video clip showing the platform packed with people up to where the escalators are.
Despite the situation, it was not until around 2.30pm that SMRT tweeted about the fault, saying that the issue would continue to affect service for the rest of the day.
It warned that journeys during the evening peak time could take up to 10 minutes longer. It also asked commuters to hold on to handrails, as they "may experience intermittent braking". It did not respond when asked why it did not make a public announcement earlier.
The protracted nature of the signalling fault - which SMRT did not elaborate on - is uncommon, as most such glitches are rectified within a few hours.
The MRT network has seen some unusual incidents in recent weeks. On Aug 15, an air-con refrigerant leak caused smoke to engulf a train at Tanjong Pagar station. Last month, there were at least two incidents where ice shards fell from air-con vents on Circle Line trains.