The gloves came off yesterday in the contest for Bukit Batok, as Prime Minister Lee Hsien Loong called out Singapore Democratic Party (SDP) chief Chee Soon Juan for being duplicitous, saying he had not changed.

The SDP hit back, accusing the People's Action Party (PAP) of mud-slinging and character assassination, and saying that attacking Dr Chee's character was "uncalled for".

Mr Lee had raised questions about Dr Chee's character and whether it made him a suitable MP. He pointed to how Dr Chee was "completely hypocritical" during his party's rally on Friday.

After several SDP members had hit out at former Bukit Batok MP David Ong, who resigned over an alleged affair, the SDP chief came on stage and said it is not his style to kick a man when he is down.

Mr Lee, the PAP's secretary-general, said Dr Chee came on stage looking "magisterial", "benign and almost beatific" after allowing party members to criticise Mr Ong.

He noted that Dr Chee also said in a recent newspaper interview that he was not sorry for anything he had done in the past.

Yet he is presenting himself as a changed man, Mr Lee said after meeting Bukit Batok residents with PAP candidate Murali Pillai .

Dr Chee did not respond immediately, but SDP central executive committee member Paul Tambyah said in a Facebook video post that it is not right to question a person's character. He said a person "is not defined by his or her actions or words" and while it was all right to criticise someone's speech, actions or policies, "attacking his character is uncalled for".

Dr Tambyah added that the SDP would not "use the same kind of language to criticise the Prime Minister, as the director of the National Neuroscience Institute". It was a reference to the institute's former head Lee Wei Ling - Mr Lee's sister - whose comments, critical of her brother, emerged online recently.

Last night, Mr Lee responded to the SDP's argument that character should not be discussed or examined in an election campaign.

Mr Lee said character went to the heart of a candidate's fitness for public office. Good character and integrity were requirements for PAP candidates and should be so for the opposition as well.

While the SDP and Dr Chee in particular would want it otherwise, Mr Lee said, anyone standing for public office should be prepared to have his past actions examined, transparently and honestly, so "voters can make informed, responsible decisions as to who is fit and best qualified to represent them in Parliament".