'We are holding a pooja (ceremonial offering) so that the former army chief who saved the country from terrorism is released from custody,' Ven Maduluwawe Sobitha, who led the meeting, said.
Fonseka was jailed for 30 months after he was found guilty by a court martial of favouring his son-in-law in military tenders. He was also found guilty in August by another court martial of engaging in politics while serving in the army and was dishonourably discharged.
Fonseka fell out with President Mahinda Rajapaksa after the defeat of the Tamil rebels in 2009. He ran against Rajapaksa in the presidential elections in January but lost. He was elected to parliament in April.
The parliament informed the courts Thursday that Fonseka had lost his parliamentary seat following his conviction.
'We have been informed that in view of the conviction Mr Fonseka has been unseated and therefore the next member eligible should be appointed,' a spokesman for the Commissioner of Elections Department said.
Fonseka's party, the Democratic National Alliance, said it would challenge the decision to expel him from parliament in the courts.
The ex-army chief faces three more cases in the civilian courts.