Workers' Party (WP) chief Low Thia Khiang has been re-elected as his party's secretary-general after fighting off an unexpected and unprecedented challenge by fellow Aljunied GRC MP Chen Show Mao.

It was the first time the opposition leader faced a contest for the post since he took it up in 2001, although talk had swirled of disquiet in the ranks during the last biennial Central Executive Council (CEC) election in 2014.

Mr Low retained his post with 61 votes to Mr Chen's 45.

Addressing reporters in high spirits later, the opposition veteran shrugged off the contest, saying it was "part of the democratic process". He also laughed off a question about whether there was a growing rift between him and Mr Chen, who he unveiled as the party's star catch in the 2011 General Election.

He said: "We all respect each other. We agreed that no matter the outcome, everyone should unite and move forward."

Mr Chen echoed this sentiment, but did not want to elaborate on why he mounted a challenge. He added that he looked forward to "the next steps" for the party under Mr Low's leadership.

The unified response was in stark contrast to what party cadres described as a very tense atmosphere earlier, once it became clear a contest would take place.

Party leaders were caught off guard and some cadres protested against Mr Chen's nomination before the voting, said party sources. After Mr Low prevailed, some of his supporters pumped their fists in the air and chanted his name when they spotted reporters gathered outside the conference room in Cecil Street.

The party had rented the room to accommodate the larger number of cadres, which some party sources had said were inducted last month to strengthen Mr Low's position.

In an unusual move, it had brought in 28 new cadres, compared with fewer than 10 in the last round two years ago.

Despite failing in his bid for the top post, Mr Chen remains in the CEC with the second-highest vote count of the day for CEC members, behind only Mr Pritam Singh, who is also an MP for Aljunied GRC.

But the two who nominated him for the post - former deputy treasurer L. Somasundaram, 53, and council member John Yam, 54 - lost their bids for a seat on the council.

Party sources said the election has purged the CEC of those agitating for a leadership change and consolidated Mr Low's hold on power.

Mr Somasundaram and Mr Yam were among four former council members who did not make the cut this time. The other two are former vice-chairman Mohammed Rahizan Yaacob, 60, and former webmaster Mohamed Fairoz Shariff, 36.

A cadre member, who asked not to be named, said: "There is a divide between those who believe in Mr Low's direction for a credible party based on a rational and responsible opposition, and those who are disgruntled and want the WP to challenge all the seats held by the PAP."

When asked whether the party's results at last September's general election had precipitated the challenge, Mr Low said no members had raised any issues on a report he had put up on the polls. The WP narrowly retained Aljunied GRC and lost the Punggol East seat.

Analysts interviewed were split on what the challenge showed.

Some, such as Institute of Policy Studies deputy director Gillian Koh, said contests for the party's top post are part of the maturation process for any political party.

"It's an opposition party that's growing and if there's a contest for the secretary-general post, it takes it a step further," she said.

Political commentator Derek da Cunha said there are bound to be some differences among political party members about policies and approach. "This should not be surprising. In fact. this should be preferred to the notion of groupthink," he added.

But National University of Singapore political scientist Bilveer Singh saw the challenge as a sign that Mr Low "does not have the full support of the party" and has detractors who do not identify with his style, policies and strategies.

He said this was despite Mr Low having "single-handedly brought the party to a national stature which no one else has been able to do thus far".

He added: "In short, there are differences and how deep they are is best indicated by the 61-45, which is a sizeable split."

Party chairman Sylvia Lim, 51, was re-elected unopposed. In all, 21 people vied for the 12 spots in the CEC. Former Non-Constituency MP and CEC member Yee Jenn Jong, 51, did not run this time.