Hong Kong's police have been granted vastly expanded powers to conduct warrantless raids and surveillance -- as well as issue internet takedown notices -- under Beijing's new national security law.

The announcement comes as major tech companies including Facebook, Google and Twitter said they were suspending requests from the Hong Kong government and law enforcement authorities for information on users.

The new provisions, disclosed late Monday in a 116-page document, remove much of the judicial oversight that previously governed police surveillance powers.

Officers will be able to conduct a search without a warrant if they deem a threat to national security is "urgent". The city's police chief has also been granted powers to control and remove online information if there are "reasonable grounds" to suspect the data breaches the national security law.

Police can order internet firms and service providers to remove the information and seize their equipment, with fines and up to one year in jail if they refuse to comply. The companies are also expected to provide identification records and decryption assistance.

Hong Kong's leader Carrie Lam has been given broad oversight over covert surveillance powers for national security cases, including communication interception, according to the document. The police chief can ask international political organisations -- including those in Taiwan -- to supply information on their activities in Hong Kong including personal data, sources of income and expenditure.

The powers are controversial because Beijing's new national security law has effectively outlawed certain political views in semi-autonomous Hong Kong, such as support for independence or greater autonomy.

Legal experts said the new surveillance powers were broad and lacked proper oversight. "The new rules are scary, as they grant powers to the police force that are normally guarded by the judiciary," barrister Anson Wong Yu-yat told the South China Morning Post.

"For example, in emergency and special circumstances police do not need a warrant under one rule, but it never explains what it means by special circumstances. They can also ask anyone to delete messages online only because it's 'likely' to be violating the law."

The national security law is the most radical shift in how Hong Kong is run since it was handed back to China by Britain in 1997. The content was kept secret until the moment it was imposed on Hong Kong one week ago, bypassing the city's legislature.

It targets crimes under four categories: subversion, secession, terrorism and colluding with foreign forces, and gives China jurisdiction in some especially serious cases. Legal analysts, critics and many western nations warn the broadly-worded categories criminalise many peaceful dissenting opinions. Beijing says the law will restore stability after a year of pro-democracy protests.