PHNOM PENH (AFP) - Cambodian authorities on Thursday intensified a crackdown on rivals of strongman premier Hun Sen, arresting two more politicians and summoning the opposition deputy leader for questioning, their party said.

The widening clampdown came a day after six opposition politicians were put in pre-trial detention on what rights groups denounced as "trumped-up" insurrection charges that carry a prison sentence of up to 30 years.

The case is linked to a demonstration on Tuesday against the closure of Phnom Penh's main designated protest site that turned violent.

Two more MPs-elect - Nut Rumduol and Long Ry - were detained on Thursday and taken to police headquarters, opposition party spokesman Yim Sovann told AFP.

"This case is totally politically motivated," he said. "Surely this will kill democracy in Cambodia." Police could not be reached for comment, but lawyers for the opposition party said the new detainees were also accused of insurrection.

Mr Kem Sokha, the deputy head of the opposition Cambodia National Rescue Party (CNRP), was also summoned by the Phnom Penh Municipal Court for questioning in relation to the case.

He was called to appear on July 25 as the party's "responsible leader," according to a warrant seen by AFP.

In a post on his Facebook page, Mr Kem Sokha appealed to supporters to closely watch the situation and "observe the use of court power as a threat to MPs-elect".

At least 40 people, mostly security guards employed by the local authorities, were injured during the Tuesday protest clashes.

The demonstrators were demanding the reopening of the capital's "Freedom Park".

The space was created as an area for people to air their grievances but has been closed since a crackdown on striking garment workers in January that left four dead.

In a statement issued before the two new arrests, US-based Human Rights Watch (HRW) called on the Cambodian authorities to "drop trumped-up charges" against the six opposition politicians charged on Wednesday.

HRW "found no evidence to suggest that any of the six organised, incited or participated in the violence," the group said in a statement.

The watchdog also urged Cambodia's donors to "take a strong stand against this latest attempt by the Hun Sen government to use the courts against opposition politicians".

The United Nations special rapporteur on the situation of human rights in Cambodia, Mr Surya Subedi, said in a statement that tolerance was crucial for "the future of democracy in Cambodia".

He also called on the government to "ensure the fair treatment of those arrested in strict accordance with the human rights standards relating to the administration of justice".

Mr Hun Sen, 61, who has been in power for nearly three decades, is regularly criticised by campaigners for ignoring human rights and crushing dissent.

The opposition has boycotted parliament since a contested election last year which triggered allegations of vote-rigging.