Sirisena, who had also been No. 2 figure in the ruling Sri Lankan Freedom Party (SLFP), quit Sri Lanka's cabinet on Friday, accusing President Rajapaska of moving the country "towards a dictatorship."
His defection -- along with the departures of three other ministers -- coincided with the setting of January 8 as the presidential election day by Sri Lanka's Election Commission.
Rajapaksa had on Thursday announced a snap election two years before his second term ends, hoping to ride popularity from past military gains.
The 69-year-old came to power in 2005 and won a second term in 2010 after the island nation's military defeated Tamil Tiger separatists, ending a 26-year-war.
Abolish presidency, says Sirisena
Sirisena on Friday vowed to abolish the powerful post of president and return to a government headed by a prime minister as existed before 1978. He also promised independent bodies to appoint judges and police chiefs.
"The 18th amendment is a serious mistake," said Sirisena, referring to Rajapaksa's recent use of parliament to change the constitution and scrap a previous two-term limit for the presidency.
Reports from Colombo said Sri Lanka's main opposition United National Party (UNP), along with rival lawmakers, had thrown their weight behind Sirisena.
"One family has captured the country's economy, wealth, administration and the management of the political party," Sirisena said, referring to senior public posts held by three Rajapaksa brothers.
Sirisena left the government with three other ministers and a lawmaker.
"After the war victory, the present government started veering in a direction that none of us expected," Sirisena told reporters while accompanied by former President Chandrika Kumaratunga.
ipj/tj (AP, Reuter, AFP)