Singapore Democratic Party (SDP) chief Chee Soon Juan yesterday questioned who his opponent is in the Bukit Batok by-election.

He said People's Action Party (PAP) candidate Murali Pillai is "not saying anything" but "everybody else" is.

"All I know is that I'm getting very confused who my opponent is," Dr Chee said to reporters after a morning walkabout in Bukit Batok.

"I thought it is Mr Murali. It is turning out to be everybody else except him."

He was referring to PAP leaders, who have in the past few days asked voters to consider a candidate's character in deciding whom to vote for.

Emeritus Senior Minister Goh Chok Tong had compared Dr Chee to a fox persuading a farmer to let him tend to the hen house, in a Facebook post on Monday.

Separately, Social and Family Development Minister Tan Chuan-Jin had listed in a Facebook post the various duties that his grassroots activists carried out, in response to Dr Chee's comments that an MP needs to do more than just greet residents "ho bo" ("how are you" in Hokkien).

"Don't see any hobos here," he wrote in a play on Dr Chee's words.

Asked about this by reporters, Dr Chee said: "I don't even want to dignify that with a response."

At his party's rally last night, Dr Chee noted that Deputy Prime Minister Tharman Shanmugaratnam had agreed that the election campaign should be clean and fair when he spoke to SDP executive committee member Paul Tambyah at the Nomination Centre last Wednesday.

But he said PAP members had "broken the promise repeatedly".

He called on Mr Murali to stop the PAP from engaging in "dirty politics" by repeatedly bringing up issues about his character.

He added that the SDP has refrained from doing the same, even though they did not agree with Mr Murali or his political views.

"I'm in charge of my campaign. If you are in charge of yours, you will ask your colleagues to stop their nonsense," he said.

His call was echoed by actress Neo Swee Lin, a surprise guest at the SDP's rally last night.

Ms Neo, who said she has known Dr Chee for five years, said she was "ashamed" of what she deemed to be "gutter politics".

She read out a petition that she said had been started by 30 people from the arts community, academia and civil society, calling on the "personal attacks and character assassination" of Dr Chee to stop.

Dr Chee said later that only those closest to him, and not the PAP, are qualified to judge his character.

"It doesn't matter who the PAP says I am. What matters is that God knows who I am, my wife and my children know who I am and, most important, I know who I am," he said.