Former Umno leader Khairuddin Abu Hassan and lawyer Matthias Chang, a former aide of Dr Mahathir Mohamad, have been barred from leaving the country.
The duo have lodged several reports in various countries regarding 1MDB.
In a Facebook post, Khairuddin said he was supposed to leave to New York and London this morning.
The former Batu Kawan Umno division deputy chief claimed the Immigration Department had blacklisted him upon instructions of the police.
"The immigration department or police did not provide me with any notice," he said.
Previously, he lodged reports in Berne in Switzerland, London and also Singapore.
"I made preparations by buying airline tickets and booked accommodation in London and New York. Now, I incurred losses after not being allowed to leave without a valid reason," he added.
Khairuddin said he went to the Immigration Department in Putrajaya this morning with Chang, but was only told that they received instructions from Bukit Aman.
Khairuddin said what is disappointing is that the Immigration Department did not give a specific reason for barring him and only cited "special reasons" and that the directive came from the police headquarters.
"I feel the authorities have abused their powers by prohibiting me from traveling abroad without a valid reason.
"Are they afraid of my actions to fight for the truth regarding the 1MDB financial scandal?" he asked, adding the authorities should have investigated those involved in 1MDB.
'Public not stupid, they're watching'
Khairuddin warned that the public are not stupid, as they are watching closely what is happening and sees this as an abuse of power by the government.
The Umno man added he had been summoned to Bukit Aman on Monday, and was told that it concerned evidence he submitted to the Berne attorney-general's office.
"Is this a crime?" he asked, adding he has the right to lodge reports in any part of the world.
He claimed the 1MDB scandal involved several Swiss banks as there are several accounts related to the scandal.
Khairuddin said he is representing many Malaysians who want to know the truth.
He warned that should the police fail to provide any reasons for the travel restrictions, he would institute legal action for abuse of power.