South Korean President Park Geun-hye faced mounting calls to step down as a record number of people at a massive rally on Saturday criticised her as unfit to rule over allegations she allowed a friend to meddle in state affairs and wield influence.

The protest rally in downtown Seoul drew more than half a million people, according to its organisers, many of whom were ordinary citizens who packed the main streets running through the city centre including a 12-lane thoroughfare.

They came with family, and students and young parents pushing strollers were among the crowd as were people in wheelchairs and crutches, in a sharp contrast from some previous rallies often dominated by militant unions and civic groups that had turned violent and clashed with police.

They chanted "Step down, step down, you must step down."