Amnesty International said Tuesday that the deaths of 25 people in clashes between Thai forces and anti-government protesters must be investigated and those responsible held accountable.

"All sides in Thailand's widening political conflict should immediately commit to ending human rights abuses," the London-based group said after Thailand's worst civil unrest in nearly two decades.

The violence on April 10, triggered by a military attempt to clear protesters from their rallying base, left 19 protesters, five members of the security forces and one foreign journalist dead, as well as over 800 injured.

"Amnesty International welcomes the Thai government's pledge to investigate promptly, effectively, and impartially the recent violence, and urges it to provide accountability for any violations by security forces as well as abuses by violent protesters," the group said.

"The military used lethal force in the operations, while some individuals among the protesters also used guns and grenades, as well as improvised weapons."

The sequence of events on the night of April 10 remains shadowy. Witnesses and video footage indicated that the violence was kicked off by mysterious black-clad gunmen who have been disowned by both sides of the conflict.

Amnesty said the Thai government was obliged to protect the lives of all those involved in the crisis "including by exercising due diligence to prevent attacks by non-state actors".

It also said that the right to freedom of assembly only relates to peaceful gatherings, and that political parties backing the protesters should "ask their supporters to refrain from further violent attacks on political opponents".

The red-shirted campaigners, who are calling for the ouster of Prime Minister Abhisit Vejjajiva, have refused to leave their sprawling new encampment in the heart of Bangkok's retail district.

Security forces armed with assault rifles swooped into the nearby financial hub on Monday to thwart plans by the Red Shirts to march there in the next step of their month-long campaign of street rallies.

(Source: AFA)