The two-hour rally is due to begin at 7pm.
The front, which arranged this summer’s marches on Hong Kong Island that attracted hundreds of thousands of participants, had said on Wednesday that they would be forced to cancel the rally if the police refused to grant them a “letter of no objection”.
The letter from the force says Saturday’s rally must come to an end by 10pm.
The group are also hoping to hold a march from Causeway Bay to Central on the October 1 National Day holiday, again depending on whether they get police permission.
The force has banned many of the protests planned in recent weeks on safety grounds, but large crowds of protesters had often turned out nevertheless.