Hong Kong's leader has denied there was any political interference in a court's decision last week to send three young protest leaders to jail.

Hong Kong Chief Executive Carrie Lam made the denial at a news conference Monday after tens of thousands of people rallied to protest the sentences given the three leaders.

The High Court overturned an earlier verdict that had allowed the three activists to avoid prison.

The activists — Joshua Wong, Nathan Law and Alex Chow — were accused of joining or leading an unlawful assembly that sparked huge 79-day pro-democracy protests in 2014.

Lam said there was no political interference in the prosecution, review or rulings handed down by the court.

Wong, Chow and Law were sentenced to six, seven and eight months in jail respectively.