Sri Lanka has been trapped in a bitter power feud since President Maithripala Sirisena replaced Wickremesinghe with the controversial strongman Mahinda Rajapakse.
Sri Lanka's ousted prime minister promised Monday to bring tens of thousands of supporters to the capital next week for a massive demonstration unless President Maithripala Sirisena reinstated him imminently.
Ranil Wickremesinghe, whose dismissal in October plunged Sri Lanka into crisis, said he still commanded the support of parliament and Sirisena had until Friday to recognise that.
Sri Lanka has been trapped in a bitter power feud since Sirisena replaced Wickremesinghe with Mahinda Rajapakse, a controversial former strongman leader.
Parliament has twice voted against Rajapakse but Sirisena has refused to reinstate his former ally Wickremesinghe -- even if he has the numbers on the floor.
The country's Supreme Court is ruling this week on whether Sirisena's sacking of parliament in November was unconstitutional.
Wickremesinghe -- who is confident of a ruling against Sirisena -- said tens of thousands of the party faithful would rally in Colombo if the president did not heed the court.
"After the court ruling, we will launch our "People Power' campaign to force the president to end the crisis," he said in a statement.
Wickremesinghe has cobbled together a majority in parliament, which has twice voted against Rajapakse.
A court of appeal last week also stripped Rajapakse and his cabinet of their authority until he could prove his right to hold office.
The war-era strongman, backed by the president, has ignored parliament, and has named a cabinet and assumed the duties of prime minister.
But the 225-member legislature has blocked his office from spending any funds, worsening the acrimony between the factions.
The chamber has witnessed violence in recent weeks as the power struggle dragged on, with brawls between warring sides.
Last week, Wickremesinghe likened Sirisena to Hitler, his strongest comment yet directed at his former ally.