CROSS-BORDER traffic at Man Kam To ground to a standstill yesterday when up to 4,000 drivers parked their container trucks on both sides of the checkpoint in protest against alleged corruption and inefficiency on the Chinese side. The protest started shortly before 10 am when one driver returning from China parked his truck across the incoming lanes. He was joined by thousands of supporters, forming a 20 kilometre queue. Drivers reiterated accusations of corruption and inefficiency among mainland customs officials at the checkpoint. They said only two of the eight counters were open and Chinese officials had demanded ''tea money'' for quick clearance. Hongkong police failed to get drivers to attend a liaison meeting, while local customs officials held a marathon meeting with a drivers' union in a Sheung Shui restaurant until late last night. The Government appealed last night to protesting drivers to remove the trucks so traffic could move when the checkpoint reopens at 7 am today.