Singapore is holding a by-election to fill a parliamentary seat vacated after an opposition Member of Parliament was expelled by his party in February.
More than 20,000 residents of Hougang constituency in the island's northeast are eligible to vote.
They will be choosing between the ruling People's Action Party (PAP) candidate Desmond Choo and Png Eng Huat of the Worker's Party (WP).
The ward has been represented by the opposition for more than 20 years.
This is a rarity in the island nation, which has been led by a strong majority PAP government for most of the last five decades since the party first came to power in 1959.
The ruling party has cast this as a local election and its candidate has focused on improvements to the residential estate.
But the opposition party maintains the poll is more than that, saying issues faced by voters in Hougang are the same as for the rest of the nation, such as a widening income gap and the need for more diverse voices in parliament.
Some observers say the poll is being seen as an indication of whether the promises of change that the prime minister made after the general election - such as engaging the people more - have taken root.
The two candidates and their supporters spent the past week campaigning, apart from a ''cooling off day'' on Friday when they were not allowed to canvass for votes.
The former MP, Yaw Shin Leong, was expelled by the WP on 15 February over allegations of an extramarital affair.
Mr Yaw had won the seat over the PAP's Mr Choo in the general election last year, with 64.8% of the votes.
The last poll in 2011 was seen as a landmark election for the city state with significant gains for the opposition, which took six of the 87 seats - their best performance since Singapore's independence in 1965.
Although the ruling party won the great majority of seats, the party's share of the popular vote slipped to around 60% - a drop from about 67% in 2006.
After months of speculation on whether a by-election would be called, President Tony Tan issued the writ of election on 9 May.
Following that, Prime Minister Lee Hsien Loong posted a statement on his recently set-up Facebook page.
''The by-election will give Hougang residents the chance to elect a new MP to serve them. I encourage Hougang voters to use this opportunity wisely, to elect the best candidate with commitment and integrity," he said.