"Whoever was involved in this incident should be brought to justice"
Filomeno Aleixo, Fretilin spokesman
Gaspa da Costa, the local police chief, said the unrest broke out following a Ramo-Horta campaign rally, when supporters brawled with youths aligned with Fretilin, the party of deposed leader Mari Alkatiri.
Filomeno Aleixo, a Fretilin spokesman, said the party did not instigate the violence and denied involvement.
"Whoever was involved in this incident should be brought to justice," he told The Associated Press.
The UN police force in East Timor said in a statement that supporters of Ramos-Horta were attacked, but did not say by whom.
Eight presidential candidates are midway through two weeks of campaigning.
Rallies and speeches are scheduled across 13 districts up until a blackout period two days before polling day.
East Timor, which became Asia's newest nation in 2002, descended into chaos one year ago after Alkatiri dismissed 600 soldiers, a move that split the armed forces into factions and later spilled over into gang warfare.
At least 37 people were killed and 150,000 others fled their homes.
Thousands of international troops were deployed to help curb the worst of the violence.
East Timor voted to break free from 24 years of Indonesian rule in 1999.
The country was administered by the UN, and until last year's crisis, which led to the overthrow of the government, had been considered a major success in nation-building.