Disgruntled Bulgarian truck drivers blocked traffic at two major border checkpoints with neighbouring Turkey on Friday to protest against what they said were Turkish restrictions to their operations.

Among those caught up in the blockade, now in its second day, was British band Depeche Mode, which was forced to cancel its concert in Istanbul on Friday because trucks carrying equipment from Bulgaria could not get through.

A queue 12 km long had built up at the main Kapitan Andreevo crossing and another around 5.5 km long at the Lesovo checkpoint.

Drivers said they had been subjected to lengthy checks by Turkish customs officials, and other unspecified requirements had been demanded.

"It is obvious they want to push us out of the Turkish and Arab transport market," driver Branimir Bonev said. "These are discriminatory measures against us by Turkish customs officials."

Bulgaria's interim government said Turkey had promised to lift most of the restrictions, but verbal assurances failed to calm angry truck owners and drivers.

"We will continue the truck blockade until we see a written agreement between the two countries," said Iliyan Filipov from Bulgaria's road transport association. "We are ready to stay here for a month if necessary."

On its website, Depeche Mode apologised to fans for the cancellation.

"The Bulgarian trucking blockade at the Bulgaria-Turkey border has prevented Depeche Mode's production trucks from crossing the border into Turkey, forcing this situation."

Cars and buses were moving normally through border checkpoints, but Bulgaria's interior ministry said 120 policemen had been sent to the crossings to maintain public order.

Hundreds of Turkish truck drivers protested in front of the Bulgarian Embassy in Ankara in June, 2008, against transit fees introduced by the Bulgarian authorities.