The 20 SMRT bus drivers assisting with police investigations into possible breaches of the law as a result of the illegal strike earlier this week, were held at the Police Cantonment Complex for at least half the day.

They were first seen reporting to the complex yesterday morning at about 10am.

Police were unable to comment further, citing ongoing investigations. It is also not clear if any of them were arrested.

One group is believed to have left the complex after midnight, returning to their dormitory in Woodlands at about 1am on Thursday.

A second group of about 10 drivers, most still in their SMRT uniforms, were seen leaving this morning at about 6.50am in three white passenger vans.

SMRT said on Wednesday that all 450 of its Chinese bus drivers were accounted for, unlike on Monday when 171 did not work, and on Tuesday when 88 stayed away.

The 20 drivers who reported to the police on Wednesday were among those who did not go to work on both Monday and Tuesday.

In Singapore, strikes are legal only if workers follow rules stipulated in the Trade Unions Act and Trade Disputes Act. Tougher provisions of the Criminal Law (Temporary Provisions) Act, however, spells out the rules for workers doing essential work to strike legally has been in place since 1955, with more categories of workers added over the years. Public bus and air transport workers made the list in 1967.

Those who flout the law, including the ringleaders, can be arrested, tried in open court and jailed for up to 12 months.

Since their employer SMRT provides an essential service and the China-born drivers did not give 14 days' notice, their strike was illegal.