Singapore's governing party faces a fight for power for the first time in 18 years.
PAP candidates face a challenge for the first time in years
Opposition parties say they have raised enough candidates to contest more than half the seats up for grabs in the 6 May vote.
It denies the ruling People's Action Party (PAP) the guaranteed victory it has enjoyed in the last three polls.
However one opposition party was hit with a lawsuit alleging defamation against the Singapore prime minister.
Prime Minister Lee Hsien Loong and his father, former Prime Minister Lee Kuan Yew, are suing the Singapore Democratic Party leader Chee Soon Juan and other party members over articles they wrote criticising the government's handling of a scandal at the National Kidney Foundation.
Opposition figures have been sued for libel in the past, and sometimes bankrupted.
'Vote without fear'
Despite greater participation by the opposition, analysts still believe the PAP, which has ruled since independence in 1965, will form the next government.
The party has been credited with turning Singapore from a tiny territory with few resources into a wealthy financial dynamo.
However critics say it is still a restrictive political model with little in the way of open democracy.
The opposition has only two seats in the current parliament.
In the last vote, in 2001, only a third of voters were offered a choice of candidate, with the opposition fielding hopefuls for 29 of the 84 seats.
This time the main opposition parties - the Workers' Party (WP), the Singapore Democratic Alliance (SDA) and the Singapore Democratic Party (SDP) - have announced they will together contest 47 seats.
WP chairman Sylvia Lim said she did not expect to topple the PAP, but urged people to "make their own assessment, look objectively at the candidates offered to them in their constituency and vote without fear or favour".
The prime minister said he welcomed the competition.
"We are eager to fight them, we're going to work hard and make sure we win convincingly," he said.
The PAP has vowed to boost the country's economy and help the poor and elderly in the wealthy city-state.