More than 20 SMRT bus drivers from China have been brought to Admiralty West Prison.

They were first seen leaving their dormitories located in Woodlands and Serangoon at about 9am on Saturday under police escort.

The drivers, believed to be among those who took part in a strike earlier this week at the Woodlands Sector 1 dorm, looked calm as they boarded the buses under the watchful eyes of police officers.

About 20 from the Woodlands were escorted by the officers into three SMRT-chartered buses, and brought to the prison.

At least five other drivers from the Serangoon dorm, were similarly ferried to the same prison, in what appeared to be a simultaneous police operation at both locations.

This latest action by the police comes after the illegal strike by mainland Chinese drivers - 171 of them did not show up for work on Monday, citing unhappiness over wages and poor living conditions. The next day, 88 were absent.

Four of the drivers who allegedly instigated the strike were charged in court on Thursday and remanded a week for further investigations.