The Colombo campaign office of defeated Sri Lankan presidential candidate Gen Sarath Fonseka has been raided by 40 policemen, his supporters say.
The say that Criminal Investigation Department (CID) personnel arrested 13 people in the raid.
The government has given almost no information on the operation, which saw a street in Colombo cordoned off.
The military excluded all journalists from the leafy street where the general's campaign office is situated.
Gen Fonseka has refused to accept his defeat in the elections.
He argues that his supporters were intimidated and the result was fixed.
The BBC's Charles Haviland in Colombo says that dozens of security forces were seen arriving in the street where Gen Fonseka ran his office from his home.
"They want to take all [my] people into custody and take them to the police station, saying that we had been planning a coup from that office," Gen Fonseka told the BBC Tamil service.
"All nonsense. Right now they are packing up all our computers, all our equipment, and they are trying to take all the staff to police station."
On Thursday the general said that he wanted to leave the country because of death threats.
As votes were being counted on Wednesday, troops surrounded him inside the hotel, where his campaign was based, on suspicion he was plotting a coup.
Opposition lawyer Shiral Lakthilaka said that dozens of CID officers escorted by commandos arrested 13 men who had been involved in the general's election campaign, all of them retired soldiers or officers.
He said they were now being held incommunicado even though police found none of the weapons or explosives that they said they were looking for.
No senior police officials were able to provide information to the BBC - one said he knew nothing, others were unavailable by telephone.
A defence official would only say an investigation was going on.
The government had earlier accused the general of planning to assassinate his victorious rival, President Mahinda Rajapaksa.
Gotabaya Rajapaksa (left) is a close confidant of the president
On Thursday Defence Secretary Gotabaya Rajapaksa, the brother of President Rajapaksa, said the government was considering taking action against Gen Fonseka.
He denied that the opposition leader was being investigated because of his decision to stand in the acrimoniously fought elections.
Mr Rajapaksa said Gen Fonseka had wrongly accused him of ordering the killings of three senior Tamil Tiger rebels as they had tried to surrender in the final stages of the conflict last May.
President Rajapaksa won six million votes compared with the four million cast for Gen Fonseka in the vote held earlier this week.
The independent Centre for Monitoring Election Violence said there were reports of irregularities but no evidence to suggest large-scale fraud.