A human rights group wants Singapore to drop the charges against four Chinese bus drivers accused of instigating a strike at transport firm SMRT.
They were charged after 171 drivers, all of whom were recruited from China, abstained from work last week over pay.
Singapore has already jailed one bus driver for six weeks and deported 29 others for staging the strike.
Strikes are illegal in Singapore for workers in essential services, unless the employer is given 14 days' notice.
However, Human Rights Watch (HRW) said that none of the conditions for a sector to be deemed essential "can reasonably apply to the situation of the Chinese bus drivers in Singapore".
It argued that, according to the International Labour Organization, restrictions on the right to strike "can only be justified in sectors that are 'essential' in the strictest sense of the term, where the interruption of that service could endanger the life, safety or health of all or part of the population, or in situations of acute national crisis".
It said the strike had not led to a mass disruption of service, with only 5% of Singapore's bus services being affected.
If convicted, the four drivers face a maximum fine of 2,000 Singapore dollars ($1,600; £1,000) as well as the possibility of up to a year in jail.
The Chinese drivers who took part in the strike say they are being paid significantly less than drivers of other nationalities and that their company-provided accommodation - which HRW says the drivers paid for - is poor and unhygienic.
Phil Robertson, deputy Asia director at HRW, said that Singaporean authorities needed to address these issues.
"Sadly, the government is drawing the wrong conclusion from the bus drivers' strike," he said.
"Rather than prosecuting migrant workers who speak up, the government should ensure that all workers have equal rights."
The drivers appeared in court on Thursday and were granted bail.
Bail was set at SG$10,000 for three of them, and at SG$20,000 for one driver who faces an extra charge for posting a comment on a Chinese social networking site urging other to join the strike.
However, reports in various media suggested they may struggle to raise the funds.