Sinn Fein has said it will complain to the police ombudsman about a confrontation outside a County Down count centre.
Mr Butler was leaving count centre after election defeat
The beaten SF candidate in Lagan Valley Paul Butler was leaving the grounds of Dromore Leisure centre on Friday when police stopped the convoy of cars in which he was travelling.
Police said they believed one of the cars had been stolen in the Irish Republic on Wednesday.
Officers checked the serial number on the car and, for several minutes, Mr Butler and supporters were involved in an angry confrontation with officers.
They said they were being harassed.
After about 10 minutes they were allowed to leave. Police said they were satisfied the vehicle had not been stolen.
The PSNI later apologised for any inconvenience. A spokesman said an error had been made in recording and checking the registration number of a vehicle during routine duties.
The incident follows Mr Butler's claims that on Thursday night, in the same car park, he was threatened by loyalists.
A PSNI spokeswoman said police had not received any report of intimidation and were not approached for assistance.