Updated at 6.00pm: Three pro-democracy legislators in Hong Kong said telephones in their homes and offices had been tapped by unknown people, a local newspaper reported on Tuesday. The legislators - former Democratic Party chairman Martin Lee Chu-ming, independent Albert Cheng King-hon and Leung Kwok-hung - said they had installed a sensor, known as a 'Bug Smasher', on their phones to detect eavesdropping, the Apple Daily reported. Mr Lee, a veteran legislator and barrister, said the device had indicated that one of his three office phones was overheard 24 hours a day, the newspaper reported. The device has its lights on when it detects the phone line is being tapped, the report said. Mr Leung, a leading pro-democracy activist, said he found that an office phone had been tapped when the counter-surveillance device was installed in March. But the three legislators said they had no idea who was eavesdropping on their phone conservations. Secretary for Security Ambrose Lee Siu-kwong said on Tuesday he would not comment on the allegations. But he said the authorities had not done anything illegal. 'The government has always strictly followed the law. We do not conduct clandestine operations for political reasons,' Mr Lee said. The allegations come as legislators are debating a government bill which proposes allowing law enforcement agencies to conduct covert surveillance. The bill suggests that law enforcement department heads may authorise officers to use phone-tapping devices to prevent or detect serious crime or protect public security. It has precipitated considerable debate. Some lawmakers think the provisions are vague. They have questioned why the term 'public security' is necessary, given that it lacks a precise definition and may broaden the scope of the powers given to officers. They also asked what was covered by public security that did not constitute a serious crime in Hong Kong.