Tensions ran high in the streets of Phnom Penh as thousands of police and military personnel were deployed to prevent organized demonstrations, the Phnom Penh Post reported,
Police set up checkpoints along national roads leading into the capital and vehicles were searched, military police spokesman Kheng Tito said.
Police also turned away people entering the city if they were suspected of heading to what could have been an organized demonstration.
Outside the National Assembly, authorities moved fire trucks to surrounding roads and parked razor wire across the street, the Post reported.
Inside the National Assembly, the 68 members of Hun Sen's Cambodian People's Party reappointed him as prime minister.
All 55 seats held by the opposition Cambodia National Rescue Party were empty, despite increasing its representation from 29 seats in the previous Parliament.
Hun Sen, 60, won the July 28 national election, albeit with a reduced majority, to extend his 28-year rule.
The Post reported what it said was "a brutal show of force" upon a peaceful demonstration that included journalists and human rights workers around the Wat Phnom Buddhist temple.
"Dozens of police and thugs dressed in civilian clothes descended on a peaceful vigil at Wat Phnom last night and set upon the roughly 20 protesters with slingshots, batons and electric prods."
At least five people were treated at Calmette Hospital for wounds.
As they left the area, according to witnesses, police and a group of young men began shooting marbles into the group with slingshots.
The Post reported rights activist Bo Chorvy saying a U.S. Embassy vehicle attempted to enter the area, but police forced it to turn around.
The parliamentary boycott came as no surprise to the government and police.
Opposition leader Sam Rainsy had been holding public gatherings, saying the CNRP would boycott the opening session of Parliament if allegations of election irregularities weren't investigated.
The opposition party's best hope for toppling Hun Sen was Rainsy, a controversial figure who had been living in exile until he returned less than two weeks before the election.
Rainsy was a member of the National Assembly in 2009 when he led demonstrations near the border with Vietnam to protest alleged Vietnamese encroachment upon Cambodian Territory.
He eventually was charged with racial incitement and destruction of property and stripped of his parliamentary immunity.
Rainsy, 64, fled overseas before being tried and sentenced in September 2010 to 10 years in jail.
When he returned in July after a royal pardon, thousands of people greeted him at the airport, although he was ineligible to run in the election.
"I have returned to rescue the country," he said.
I think we are entering a new phase in Cambodian history," Rainsy told Time magazine by phone upon his return. "It is the beginning of something like the Arab Spring."
Under Hun, Cambodia has been opening up to foreign investment, but the country's rights record remains desperate, Human Rights Watch said in a July report.
"The human rights situation in Cambodia deteriorated markedly in 2012 with a surge in violent incidents, as the ruling Cambodian People's Party prepared for national elections," HRW said.
"Violence involving state security forces occurred amid increasing land-taking by powerful business and security interests."
There also was growing labor unrest "due to dissatisfaction with an economic policy that relies heavily on state authorities' often-corrupt promotion of unbridled foreign investment."
Foreign investors were being granted "economic and other land concessions, which continued despite the government's May 2012 announcement of a moratorium," the HRW report said.
Hun's government also has faced international criticism over the ongoing trials of suspected Khmer Rouge leaders.
Earlier this month HRW claimed the government's refusal to pay local staff at the Khmer Rouge tribunal is "the latest attempt to undermine efforts to bring former Khmer Rouge leaders to justice."
More than half of local Cambodians on the government payroll at the Extraordinary Chambers of the Courts of Cambodia went on strike to protest the government's failure to pay their salaries since May.
The strike includes interpreters, translators and judicial and technical staff.
"The failure to pay staff threatens efforts to finish the first segment of the trial of former Khmer Rouge leaders Nuon Chea and Khieu Samphan," HRW said.
Closing statements in the case are scheduled for October and a verdict is expected in the first half of 2014.