By Uditha Jayasinghe and Devjyot Ghoshal

COLOMBO, July 12 (Reuters) - 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, as anger rises against the powerful family for a debilitating economic crisis.

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.

The Sri Lanka Immigration and Emigration Officers Association said its members declined to serve Basil Rajapaksa at the VIP departure lounge of the Colombo airport.

"Given the unrest in Sri Lanka, immigration officials are under tremendous pressure to not allow top-level people to leave the country," K.A.S Kanugala, chairman of the association, told Reuters.

"We are concerned for our security. So until this issue is resolved, the immigration officials working at the VIP lounge decided to withdraw their services."

Pictures of Basil Rajapaksa at the lounge were reported by local media and widely shared on social media, with some expressing their anger at his attempts to leave the country. Basil Rajapaksa could not be immediately reached for comment and a close aide declined to give details.

A top official in the ruling party said on condition of anonymity that Basil Rajapaksa was still in the country.

The Rajapaksa family, including former Prime Minister Mahinda Rajapaksa, has dominated the politics of the country of 22 million for years and most Sri Lankans have blamed them for their current misery.

The tourism-dependent economy was hammered badly by the COVID-19 pandemic, as were remittances from overseas Sri Lankans, while a ban on chemical fertilisers damaged farm output. The ban was later reversed.

The Rajapaksas implemented populist tax cuts in 2019 that affected government finances while shrinking foreign reserves curtailed imports of fuel, food and medicines.

Petrol has been severely rationed, and long lines have formed in front of shops selling cooking gas. Headline inflation hit 54.6% last month, and the central bank has warned that it could rise to 70% in the coming months.