If convicted, all six, including long-time reformist Akilisi Pohiva and Clive Edwards, a former cabinet minister, will automatically lose their seats in parliament.
The November 2006 riots began after parliament went into recess for the year without voting on proposals for constitutional change.
The government later agreed to hold new elections in 2008 in which a majority in parliament would be elected directly by popular vote, leaving the rest for noble families and royal appointments.
|King George Tupou V is seen as |
being reform minded [EPA]
Before Thursday's polls seven of the nine seats allocated to commoners in the 32-member parliament were held by reformists.
Tonga's only woman legislator was not re-elected.
Pita Vuki, the registrar of electors, said the initial count showed just under half of Tonga's 67,000 eligible voters had cast a ballot on Thursday, well below the 60 per cent voter turnout at the 2005 election.
Observers say Fred Sevele, the prime minister and the first elected commoner appointed to the post, will retain his job under the reform-minded King George Tupou V.
On Wednesday nobles gathered at the king's palace in the capital, Nuku'alofa, and elected nine among them to fill nobles' seats in parliament.
Meanwhile Tonga's media council expressed concern over reporting restrictions placed on the country's public broadcaster during the elections, a move the government said was necessary to ensure balance and objectivity.
The island nation which sits halfway between New Zealand and Hawaii depends on agriculture, fishing, remittances from Tongans abroad and foreign aid to support its people.
According to the World Bank, half of Tonga's 108,000 people live below the poverty line.