South Korean police were put on high alert Saturday in the port of Busan as thousands of protestors planned to hold a night-time march in support of striking workers, a police spokesman said. More than 7,000 riot police will be deployed to prevent the march -- through the centre of the southern city to the shipyard of Hanjin Heavy Industries and Construction Co. -- erupting into violence, the spokesman said. Activists said more than 180 busloads of protesters from across the country were to gather outside the Busan railway station before they start marching to the shipyard, some four kilometres (2.5 miles) away. \"We would intervene to stop the march if it disturbs traffic flows,\" the spokesman told AFP. The march is to support Kim Jin-Suk, a 52-year-old woman who has been holed up since January on top of a 35-metre (115-foot) high giant crane at the shipyard, demanding the company call off massive layoffs. Police have said they would crack down hard on any violence. Last month, 22 protestors were arrested when labour activists clashed with security guards at the shipyard. In December last year, some 900 members of the shipyard\'s labor union went on strike in protest at massive layoffs that affected 400 jobs. The company responded by locking the shipyard. The strike and lockout lasted 190 days until the labour union, apparently fearing that police might storm the shipyard to break the strike and arrest union leaders, reached a deal with the management on June 27. Many of the striking workers returned to work but tens of others have been continuing a sit-in, with Kim perched up on top of the giant crane. They say the agreement failed to settle the issue of layoffs. For many workers in South Korea, losing their job is considered an economic death sentence because of weak social safety nets. The sit-in protest drew growing public attention, thanks to social network sites including Twitter, although the country\'s mainstream news media largely ignored it. Armed with a smartphone powered by solar-charged batteries, Kim Jin-Suk has been Tweeting with her supporters from the crane. The company has cut off electricity to the giant crane where she has been living for six months