Hong Kong police have arrested the chief editor and four other senior executives of the pro-democracy Apple Daily newspaper under the territory's national security law on suspicion of collusion with a foreign country to endanger national security, according to media reports.

Local media, including the South China Morning Post and Apple Daily, reported national security police arrested Apple Daily's chief editor Ryan Law on Thursday morning.

Others arrested included Cheung Kim-hung, the CEO of Next Digital, which operates Apple Daily, as well as its chief operating officer and two other editors.

The government said in a statement on Thursday that Hong Kong's National Security Department had arrested five directors of a company for "suspected contravention" of the national security law.

Four men and one woman aged between 47 and 63 were arrested on suspicion of collusion with a "foreign country or with external elements to endanger national security" according to the statement, which did not name those arrested.

More than 200 police officers entered Apple Daily's offices on Thursday morning with a warrant to carry out a search, local media reported.

The government said in a statement police had a court warrant and searched a media organisation on Thursday for evidence of a suspected violation of the national security law.

It is the second time Hong Kong police have conducted an operation on Apple Daily. Founder Jimmy Lai and other executives were arrested last year on suspicion of national security law violations or fraud.

Lai is currently serving a 20-month prison sentence for his role in unauthorised assemblies in 2019, during a period when Hong Kong saw massive anti-government protests. Last month, authorities froze Lai's assets and shares in Next Digital.