From his jail cell in Ukraine's capital, opposition leader Mikheil Saakashvili is calling on supporters to rally for the impeachment of the president and has declared a hunger strike.

Saakashvili, the former governor of the Odesa region who was stripped of his citizenship this summer, was arrested Friday night. He hasn't been formally charged, but prosecutors say he colluded with Ukrainian businessmen tied to Russia to topple President Petro Poroshenko.

Saakashvili rejects the allegations, pointing to his long record of opposition to Russian President Vladimir Putin.

Saakashvili must be brought to court for arraignment within 72 hours of arrest. No day for that has been announced. A spokesperson for the prosecutor-general's office, Andrei Lysenko, said Saturday that officials will likely ask for him to be held under house arrest after his court appearance.

He also faces the possibility of being sent back to his native Georgia, where he faces charges of abuse of office from his years as president from 2004 to 2013.

Saakashvili was the key figure of the 2003 Rose Revolution protests that forced Eduard Shevardnadze to resign the presidency. Saakashvili was elected the next year to replace him. During his tenure, he earned wide admiration for anti-corruption efforts, including revamping the police force, but critics said the headstrong leader showed a growing authoritarian streak.

He left Georgia in 2013, and in 2015 was named by Poroshenko to be Odesa governor. The next year, Saakashvili resigned from that post, claiming Poroshenko and other officials were impeding reforms in Odesa and he became a strong critic of his former patron.

Supporters helped him cross border

Georgia stripped his citizenship after his move to Ukraine, and Poroshenko this summer rescinded his Ukrainian citizenship, leaving Saakashvili stateless. He was out of the country when he lost Ukrainian citizenship, but forced his way into Ukraine in September, barging across the border with Poland, helped of a crowd of supporters.

Saakashvili, centre, appears with some of the supporters in downtown Kiev who freed him from a police van last Tuesday. He was arrested again late Friday. (Sergei Chuzavkov/AFP/Getty Images)

His lawyer, Ruslan Chernolutsky, said Saturday that Saakashvili had written a statement in jail saying "don't be afraid of anything and boldly go to Sunday's peaceful demonstration" for Petroshenko's impeachment.

He also said Saakashvili would refuse food while in detention.

Saakashvili's detention and his call for a protest on Sunday raise tensions in a country shaken by two massive protest uprisings in this century. Poroshenko became president in the wake of bloody 2014 protests that drove his Russia-friendly predecessor to flee the country.

Saakashvili supporters have set up several dozen tents in a protest encampment near the parliament building. But police have not moved to disperse it, possibly concerned that such action would only inflame tensions.