SINGAPORE (AP) — Singapore responded to its first strike in nearly three decades with riot police and strident official criticism of the disgruntled Chinese immigrant workers, highlighting strains from an influx of foreign labor.

Many of the 171 striking bus drivers returned to work Wednesday after a government minister warned them they had "crossed the line" and riot police were stationed near their hostel. They went on strike Monday in protest at being paid nearly a quarter less than Malaysian bus drivers who work for the same Singapore transport company.

Strikes are almost unheard of in Singapore where the ruling party has been in power since 1959 and maintains strict control over political dissent. The last strike was in 1986 by shipyard workers.

As the city-state grew wealthier over the years, its citizens increasingly spurned menial, low status work and the government, concerned about remaining competitive with lower cost countries in Asia, needed a solution. The island of 5.2 million people now relies on hundreds of thousands of immigrants from countries such as Indonesia, Bangladesh and China who work as maids, construction workers and other occupations deemed unappealing by many locals.

The influx has strained public services and sparked a backlash, particularly among low-income Singaporeans, by keeping wages down while the growing numbers of expatriate professionals working for global companies based in the city have pushed up housing and other costs.

The government is "losing the ability to feel the pulse of the public and react accordingly," said commentator and former newspaper editor P.N. Balji who characterized the strike as a "huge embarrassment" for Singapore. "This inability, if not tackled quickly, can only damage the country's jealously-guarded reputation in the long run."

The city-state's pliant workforce and reputation for political stability helped it attract significant foreign investment in manufacturing and other areas since the 1970s, transforming the island into a major port and oil refining center as well as base for financial services and manufacturing of electronics and pharmaceuticals.

On Monday, the Chinese drivers who are paid Singapore dollars 1,075 ($879) a month compared with S$1,400 for a Malaysian driver, refused to board a shuttle bus to take them to work. Riot police and four police special operations vehicles were ordered to nearby their dormitory while management of SMRT Corp., the bus and commuter rail company, tried to convince the drivers to return to work. About half continued the strike Tuesday.

"They should reflect on our behavior and investigate why we have reacted in such a way," state TV quoted a driver it didn't name as saying.

Government reaction was swift.

Acting Minister for Manpower Tan Chuan Jin announced that the strike was illegal because it disrupted an essential public service. He said the government had "zero tolerance" for unlawful behavior and police were investigating the workers.

"By taking matters into their own hands the drivers have clearly crossed the line. These workers have disrupted public transport services and Singapore's industrial harmony," Tan said.

SMRT said the strike affected about 5 percent of its bus services.

Singapore's ruling People's Action Party won 60 percent of the vote in May elections, its lowest share of the vote since 1965, as rising living costs and the influx of foreigners caused some of its support to ebb to the fragmented opposition.

The government says it has been restricting growth in the number of immigrants through measures such as increasing its foreign worker levies. A government report released in June showed the number of foreign residents rose to 1.49 million from 1.39 million the year before.

SMRT said the difference in pay between the Chinese and Malaysian bus drivers was due to the Malaysians being permanent employees.