Sri Lankan immigration officials said on Tuesday they prevented the president's brother and former finance minister Basil Rajapaksa from flying out of the country.

It was not immediately clear where Rajapaksa, who also holds U.S. citizenship, was trying to go. 

He resigned as finance minister in early April as street protests surged against shortages of fuel, food and other necessities and quit his seat in parliament in June.

His elder brother Gotabaya Rajapaksa will resign as president on Wednesday to make way for a unity government, after thousands of protesters stormed his official residence on Saturday demanding his ouster. 

The president has not been seen in public since Friday and his whereabouts are unclear.