HONG KONG (Reuters) -- Hong Kong lawmakers passed on Sunday a contentious law on covert surveillance that rights activists and pro-democracy politicians fear could erode privacy rights and allow the government to spy for political purposes.

The Interception of Communications and Surveillance Bill will regulate how law enforcement agencies in the former British colony now ruled by China monitor private communications by tapping telephones or screening e-mails.

The 60-member legislature, dominated by pro-government lawmakers, passed the bill 32-0 after pro-democracy legislators walked out in protest. The vote came after a marathon debate stretching over five days.

While the government says safeguards have been built into the new law -- such as the need for law enforcement officers to obtain warrants for covert surveillance from a panel of judges in secret courts -- critics say they are not enough.

Law Yuk Kai, director of Hong Kong Human Rights Watch, said the law is riddled with loopholes that will give the government broad surveillance powers to target individuals.

"If there's one thing I'm most concerned about, it's that the law will be used for political purposes," Law said.

Secretary for Security Ambrose Lee assured lawmakers earlier that the new bill would not be used to target political opponents, but he refused calls to put this into writing.

"If they've made such a guarantee, why not write it in the law then? This would give a much better assurance to the public," Law said.

The new law has also drawn fierce opposition from the legal community because it will allow law enforcement agencies to bug lawyers and their clients, which effectively strips the right of clients to confidential legal advice.

Pro-democracy legislators had called for some 200 amendments, which sought to introduce key safeguards in the bill and better shield journalists and lawyers from surveillance.

Ironically, the call for a bill to regulate government spying was made about 10 years ago by democrat lawmakers, who sought to put a stop to the widespread power enjoyed for decades by law enforcement agencies to snoop on residents.

The government did not respond until recently, after it lost several commercial fraud cases when the unauthorized nature of how it obtained evidence was exposed in court.

Since China took back Hong Kong in 1997, democratic forces have criticized Beijing for undermining civil, press and legal freedoms and not honoring its promise of giving the city a high degree of autonomy.