The young leaders want greater autonomy and changes to the way Hong Kong is governed by China. Among those elected is Nathan Law, 23, who helped lead the "Umbrella Protests" in 2014 for self-determination, the BBC reported.
Despite the gains, China's supporters will continue to hold the majority of seats on the 70-seat council as the pro-establishment camp won 41 seats, according to Hong Kong's Electoral Affairs Commission on Monday evening.
The Democratic Alliance for the Betterment and Progress of Hong Kong (DAB), won 12 seats in total, Xinhua news agency reported.
The Sunday election is the first in the territory since the 2014 unrest, when central areas of Hong Kong were paralysed for weeks by mostly young protesters demanding greater democracy for the territory amid concerns that Beijing was increasingly interfering in the politics of the former British colony, breaking the "one country, two systems" agreement.
More than 2.2 million eligible voters casted their ballots at hundreds of polling stations with a turnout rate of 58 per cent.
Both the total amount of voters and the turnout rate are the highest in the LegCo election since 1997.
Hong Kong's Legislative Council has 70 seats, with 35 returned by geographical constituencies through direct elections, and the other 35 by functional constituencies.
The geographical constituencies include Hong Kong Island, Kowloon West, Kowloon East, New Territories West and New Territories East, where 213 candidates contested 35 seats.
For the functional constituency election, there are 55 validly nominated candidates (excluding candidates of the District Council (second) functional constituency), and 43 of them ran for 18 seats in the functional constituencies. The remaining 12 candidates in 10 functional constituencies returned to the LegCo uncontested.
In addition, 21 candidates contested five seats in the District Council (second) functional constituency, commonly known as the "super seats".
There were nearly 3.78 million Hong Kong residents registered as eligible voters for the election of the sixth term LegCo, with an increase of about 310,000 voters from the last LegCo election in 2012.
The LegCo is the legislative body of the Hong Kong SAR. Each term of office of a legislator is four years, except for the first term when it was set to be two years, Xinhua news agency added.
The fifth term of LegCo began on Oct 1, 2012 and will end on Sep 30, 2016. The term of office of the newly-elected sixth LegCo members will begin on Oct 1.
Before nomination for the election, Hong Kong's Electoral Affairs Commission demanded that all the candidates must endorse a statement promising to support the Hong Kong Basic Law and be loyal to the Hong Kong SAR.
