Stuart Melchor, 35, was left with seven deep bites on his thigh, and several wounds to his head after the incident outside a wholesaler, Blue Gem, in north-west London.

He believes the assault may have been racially motivated. "I'd like to say there was some other possible explanation but I've gone over it in my mind time and time again and I can't think of one. It's bad enough in this job having to worry about the villains without having to look out for the police as well."

Since the incident, which took place in the early hours of March 11, Mr Melchor, a father of two, has suffered from severe headaches and sleeplessness, and now walks with a limp. He has filed an official complaint with the Metropolitan police.

Police claim Mr Melchor was set upon by the dog after its handler ordered him to stand still and he failed to do so. "The handler, believing the man may have been a suspect trying to make off, entered the unit himself and instructed the dog to restrain the man."

The police say he was bitten only once although pictures taken shortly after the incident, which have been seen by the Guardian, reveal several bite marks.

Mr Melchor was woken shortly before midnight by Shorrock Alarms informing him that the alarm at his workplace had gone off and was told that the police were on the way. He left home 15 minutes later and drove the short distance to work where he saw a suspicious car in the car park.

He asked the driver what he was doing and was told he was "waiting for a friend". He asked the driver whether he had seen the police and was told they had not arrived.

He then went to check the premises. He spent about 20 minutes checking the aisles and the offices, satisfied himself that there was no intruder and made his way out of the building.

As he opened the door to get out he was pounced on by several policemen and one dog. "I was pushing the door open and then I just heard this dog bark and a rush of people coming towards me. The dog was on my thigh and there were two policemen on each arm. I kept shouting: 'What have I done ... I'm just trying to do my job.'

"But they just told me to shut up and keep quiet and were hitting me with their truncheons. It all went on for about four minutes."

After Mr Melchor had broken down in tears, one policeman asked him whether the keys to the building, which had been beaten out of his hand, were his.

It was only then that he realised that the man he had seen earlier in the car was a plainclothes policeman. The police told Mr Melchor that he had come under suspicion because burglars often waited for the police to leave before returning to a place where the alarm has gone off and breaking in again.

"They could have asked me who I was or done a check on my car. I've been doing this job for over nine years and have dealt with the police several times without any problem. But this time they just saw a black guy and presumed that I was a thief."

Lying in a pool of his own blood and visibly distressed, Mr Melchor refused to accept the police explanation. "Don't talk to me now. You beat me black and blue and now you want to talk," he told the police after the attack. Police say they were "satisfied as to his bona fides" once he "identified himself as an employee".

An ambulance was called and Mr Melchor was taken to the Central Middlesex hospital where he stayed overnight and was given four stitches in his head.