The opposition Tamil National Alliance (TNA) says a stone-throwing mob disrupted a meeting they were holding in the town of Kilinochchi.
They say the same group had attacked their vehicles and damaged building as police looked on.
"Although there were police officers at that place... they took no effort to quell this attack," the TNA said in a statement.
A military spokesman denies security forces were involved in the most recent attack against the TNA.
Ruwan Wanigasooriya says police "successfully dispersed the crowd within about an hour, thus preventing the situation from escalating".
The accusation comes as racial tensions rise in the country, after Buddhist extremists attacked a Muslim-owned business.
A mob attacked the Muslim-owned clothing store and warehouse just outside the capital Colombo last week, in what the main Muslim party in the ruling coalition calls a "sequel" to an ongoing hate campaign against minority Muslims and other religious minorities.
Muslims make up about 10 per cent of Sri Lanka's population of 20 million people and are the second largest minority behind the mostly Hindu ethnic Tamils.
The majority of the population is Sinhalese, most of whom are Buddhist.
Sri Lanka is emerging from nearly four decades of ethnic war and tensions between different ethnic groups remain strained.
Police have arrested four men and were looking for more suspects in relation to the attack on the Muslim-owned clothing store.