LONDON - At least six people were seen on the ground after a white van plowed into people on London Bridge, a witness told Reuters.The witness, who was doing a night photography class on London Bridge, said he saw the van veering on and off the pavement on the bridge."It looked like he was aiming for groups of people. I froze because I didn't know what to do," Mark Roberts, a 53-year-old management consultant, told Reuters. "It was horrendous.""I was thinking: Which way do I run to get away?" he said, adding that the van then veered onto the other side of the bridge before hitting a girl. "I immediately thought this was the Westminster attack all over again."On March 22, Khalid Masood plowed a rented car into pedestrians on Westminster Bridge and stabbed a policeman to death before being shot dead.Immediately after Saturday's London Bridge attack, which Prime Minister Theresa May has said is being treated as a potential terrorist attack, Roberts said he saw six people on the ground."I could see six people on the floor not moving. And there were groups of people helping those people on the ground," he said. "After that I heard what sounded like gunfire from Borough Market."