SINGAPORE (Reuters) - An ethnic Chinese Singapore trade union executive was sacked after she posted expletives-laden comments about Malay weddings on Facebook, remarks so offensive they prompted the prime minister and other politicians to complain.

Amy Cheong, until Monday an assistant director at the National Trades Unions Congress, had asked how many (expletive) days did Malay weddings go on for at the foot of public housing blocks.

"(Expletive)!!!! Pay for a real wedding u (expletive), maybe then the divorce rate wont be so high! How can society allow ppl to get married for 50 bucks?"

Prime Minister Lee Hsien Loong said on Facebook he was shocked to hear of the outrage, illustrating how racial tension remains a key concern in the city-state.

"The comments were just wrong and totally unacceptable," he said.

Law Minister K. Shanmugam agreed. "Her comments reflect a deep seated racist attitude coupled with contempt for those who are less well off, or who wish to spend less," he said on his Facebook page. "There are deep fault lines in our society, based on race/religion."

The National Trades Unions Congress is an umbrella trade union affiliated to the ruling People's Action Party and is headed by Lim Swee Say, a minister without portfolio in the Cabinet.

Lim announced Cheong's sacking on Monday.

Ethnic Chinese make up about three-quarters of Singapore's resident population, with Malays making up another 13 percent and Indians 9 percent. A large number of foreigners also live or work in the city-state.

Some Singaporeans felt Cheong's dismissal was rather harsh.

"Although we cannot take her action lightly, we also cannot over-punish her," Noor Mohamed Marican, a legal adviser and council member of Singapore's Inter-Religious Organisation, told the New Paper.

"Forty plus years of multi-racial policy has not removed deep racial/religious fault lines. One reason they fester in our hearts is because we do not talk about them, for fear of saying the wrong thing and inviting a heavy-handed response," Bill Dungya wrote on Law Minister Shanmugam's Facebook page.

(Reporting by Kevin Lim; Editing by Nick Macfie)