"I will ensure a government which respects justice and equality for all," Sirisena said.
Sirisena, the former Health Minister and the General Secretary of Rajapaksa's freedom party was last week named the opposition common candidate to challenge Rajapaksa.
His defection caused a division in the ruling UPFA coalition with as many as 10 parliamentarians switching their allegiance to the opposition.
Sirisena also aims to abolish the executive presidential form of government and restore parliamentary democracy.
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"It will be a national government with all parties. After implementing reform we can return to our own political agendas and party politics," Wickremesinghe said.
Wickremesinghe said if Rajapaksa wins a third term there will not be space for political parties and the island could be affected by acts of violence.
The presidential election will take place on January 8 as Rajapaksa called the election two years ahead of schedule in an apparent attempt to seek fresh mandate before his party's popularity tumbles further, after dropping over 21 per cent in September local elections.