Hikers have been warned to be on guard after a sharp rise in the number of illegal immigrants found hiding along popular walking routes. In the past four days, Marine Police have arrested at least 31 illegal immigrants off Sai Kung who had arrived by boat from Guangdong. In the latest swoop, seven women and a man were caught near the Clear Water Bay Country Club on Saturday night. Each had paid between $2,000 and $4,000 for the speedboat trip. The arrests follow a series of swoops on illegal immigrants landing by boat off the country park in the past fortnight. 'There has been a noticeable increase in illegal immigrant activity in recent days and weeks, largely because of the time of year,' a police source said. 'The monsoon season is over, winter is not yet here and sea conditions are good for people to attempt to make it over to Hong Kong. 'And Sai Kung Country Park is an ideal gateway with a million and one places to hide.' Popular hiking paths have been pinpointed by the hikers' Adventurists Society as prime spots for illegal immigrants, including the MacLehose Trail leading to Tai Long Wan and Chek Keng. 'We advise people not to hike on their own and bring their mobile phones,' society chairman Shek Tin-lun said. 'But we oppose the carrying of weapons or any attempt to confront illegal immigrants.' A woman hiker, who asked not to be named, said: 'I was walking on a hillside with my boyfriend on Thursday from Tai Long Wan to Wong Shek Pier after dusk because we had missed the boat. 'And as we walked around a bend we saw men crouching in the bushes. 'We were horrified because we thought they might attack us but we walked past them quietly and when we saw they weren't following us we ran.' Despite the scare, police figures reveal a drop of almost 15 per cent in the number of illegal immigrants arrested this year. In the first eight months it fell to 7,792, down from 9,142 in the same time last year.