Activists expressed outrage Wednesday over Singapore's crackdown on Chinese bus drivers who staged the city-state's first industrial strike in 26 years to demand better pay and conditions.

The two-day work stoppage last week at state-linked transport firm SMRT, declared illegal by the Singapore government, has resulted in the deportation of 29 drivers and a six-week jail term for one driver.

Four other arrested drivers, who have been remanded for a week, are expected to be produced in court on Thursday, with each facing a maximum one-year jail term and a possible Sg$2,000 ($1,640) penalty if found guilty of involvement in the strike.

The Asian Forum for Human Rights and Development, which represents 47 non-governmental organisations from 16 Asian countries, urged the release of the jailed and remanded drivers and demanded that charges against them be dropped.

"We condemn the Singapore government's criminalisation of the exercise of fundamental rights by the bus drivers who went on strike," said the Bangkok-based group's executive director Yap Swee Seng.

"The swift and harsh actions overlook the bases of their complaints about wage discrepancies and poor housing."

The crackdown was also denounced by dozens of labour rights activists in Hong Kong who protested outside the Singapore consulate, with minor scuffles breaking out with security guards as they tried to enter the property.

The protesters in the southern Chinese city called on Singapore to drop the charges against the strikers and free the jailed driver.

Alex Au, treasurer of labour rights group Transient Workers Count Too (TCW 2) in Singapore, expressed dismay at the "heavy-handedness involved in prosecuting five workers and deporting 29 others".

Au told AFP the government's "zero tolerance" for illegal strikes implies that it was "not prepared to recognise that the workers had legitimate grievances".

SMRT has promised to look into the Chinese strikers' demands, fumigate their bedbug-infested dormitory rooms and find them better housing but the government has vowed to take "firm action" against any future illegal strikes.

Last week's strike, the first in Singapore since 1986, has highlighted the country's heavy dependence on migrant labour to drive its economic growth amid a labour shortage resulting from falling birth rates.

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