Hong Kong police arrested on Wednesday a columnist for pro-democracy tabloid Apple Daily on suspicion of conspiring to collude with a foreign country or foreign forces, in another hit to the besieged newspaper.

The latest arrest came after police froze assets of companies linked to the newspaper and arrested five executives last week, moves that are likely to force the newspaper to shut down this week.

Police have said dozens of Apple Daily pieces may have violated the security law, the first instance of authorities taking aim...