M1 customers found they could not surf the Web, make calls or send text messages from as early as 6am on Tuesday morning.
The telco's 3G network was down due to a power problem that affected one of its mobile network switches, according to an M1 Facebook post, which first appeared at around 6am.
The telco said it was in the process of restoring service and apologised to customers. In a second Facebook post at 11am, it advised customers to switch their devices from the 3G to 2G network.
Meanwhile, angry users flooded the M1 Facebook page with comments, with service outages reported in many areas, including Choa Chu Kang, Ang Mo Kio, Yio Chu Kang, Yishun and Jurong West. Users claimed they could not make calls or send text messages even after switching to 2G with some asking for compensation from M1.