Four mainland Chinese bus drivers accused of instigating Singapore's first strike since the 1980s were charged on Thursday with criminal offences that could land them in prison.

The four men, aged from 32 to 39, were taken to court one day after mainland Chinese drivers at state-linked transport group SMRT ended a two-day work stoppage aimed at securing better pay.

If found guilty of involvement in an illegal strike, they could be jailed for up to a year or fined a maximum of Sg$2,000 ($1,640) -- the equivalent of two months' wages for a driver.

The charge sheet stated that the four "did engage in a conspiracy" to "instigate workmen employed by SMRT Buses Ltd" to take part in a strike on Monday and Tuesday.

The drivers, looking grim in T-shirts as they were brought to court, were arrested on Wednesday and Thursday and remanded in custody for a week after being charged.

One of them, 32-year-old He Jun Ling, also faces a second charge, for posting a statement on a Chinese website urging drivers to fight for their dignity by refusing to board shuttle buses from their dormitory to a depot.

Riot police were on standby outside the workers' dormitory during the strike but no violence took place. A total of 171 drivers joined the protest on the first day, with the number falling to 88 on the second day.

The strikers were contract workers who did not belong to any union and were questioning why they were being paid less than their Malaysian counterparts for the same work.

SMRT, which is 54 percent owned by state investment firm Temasek Holdings, has had to hire bus drivers from China and Malaysia due to a chronic local labour shortage.

Asked if more arrests would be made, a state prosecutor said it would depend on the outcome of ongoing police investigations. Twenty strikers have been called in by police so far for questioning.

The drivers, who issued no strike declaration, went back to work on Wednesday after the government declared the walkout illegal and warned of a crackdown.

Strikes are illegal in Singapore for workers in "essential services" such as transport unless they give 14 days' prior notice and comply with other requirements.

Before the arrests were announced, the Chinese embassy in Singapore said on its website that it hoped the drivers' legal rights would be respected, but it also urged the workers to respect Singapore law and air their concerns through proper channels.

Singapore's last strike took place at a shipyard in 1986.

SMRT says it pays "competitive wages" and provides housing, utilities and worksite shuttles to its Chinese drivers. But it vowed to look into their grievances.

Unlike the Chinese, most Malaysian blue-collar workers in Singapore commute daily across a causeway linking the two countries.