Mohamed Nasheed said he entered into a pact with three other parties to wrest control of the 85-member national parliament currently held by President Abdulla Yameen's Progressive Party of Maldives (PPM).
Nasheed said former president Maumoon Abdul Gayoom, a half brother of Yameen, signed an agreement at the weekend vowing to work together to secure, among other things, freedom for those convicted of politically motivated charges.
"We are going to prove our majority in parliament on Monday," Nasheed told AFP during a visit to Colombo.
He said the objective was to strengthen democratic institutions and ensure that 2018 elections are free and fair.
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Last month, Nasheed said he would return to run for election as president of the troubled Indian Ocean nation, despite facing jail after a controversial conviction on terror-related charges.
He became the Maldives' first democratically elected president in 2008, but was narrowly defeated in 2013 elections by Yameen.
Nasheed was later jailed on terrorism charges he says were politically motivated. He has lived in exile for the past year after Maldives authorities gave him leave to travel to London for medical treatment.
But the former leader expects the restriction to be lifted in response to international pressure. Nasheed's international legal team includes high profile human rights lawyer Amal Clooney.
A UN panel has ruled that Nasheed's imprisonment was illegal and ordered the regime to pay him compensation.
A crackdown on political dissent in the nation of 340,000 has dented its popular image as an idyllic island paradise in recent years.
Almost all key opposition leaders and a number of ruling party dissidents have either been jailed or fled into exile since Yameen took office in a controversial run-off election against Nasheed.