Police in the Maldives are questioning four people in connection with the murder of an MP late on Monday night.
Dr Afrasheem Ali - whose party is a member of the governing coalition in the Indian Ocean archipelago nation - was stabbed to death outside his home.
The authorities have not named those held, but the former president says a party colleague of his is among them.
The Maldives has been deeply polarised ever since Mohamed Nasheed resigned in February, in what he says was a coup.
He left office in the midst of a police and military rebellion, but the security forces and his successor deny claims that he was forced out.
Mr Nasheed told the BBC's Charles Haviland in Male that one of those detained on Wednesday was a young woman whom he called a very active member of his Maldivian Democratic Party (MDP).
Describing the murdered MP as his friend, he said the woman's arrest was "comical" and a sign that innocent people were being blamed.
The MDP also says that a male activist is being held and that it fears being falsely framed for what it called the "tragic and gruesome murder", our correspondent reports.
Liberal activists say they think the killers were Islamic radicals who disapproved of Dr Afrasheem's moderate Islamic scholarship.
Meanwhile, the courts have ordered police to bring Mr Nasheed to trial on Sunday in a case in which he is accused of illegally ordering the arrest of a judge while president.
On Monday he ignored the scheduled court hearing and set off on a party campaign tour in outlying islands, forcing the case's postponement.
Mr Nasheed, who won the country's first democratic elections in 2008, says the charges against him are politically motivated.
His supporters hail him as a reformist moderate but his critics say he overreached his powers while in office.