King Norodom Sihamoni issued a decree reappointing Mr Hun Sen, giving the royal seal of approval to the results of July's general election, and the long-serving premier is expected to outline his new government's policies to parliament on Tuesday.
Security at the National Assembly and elsewhere in the capital, Phnom Penh, was tight, with many roads blocked off after opposition protests last week in which one man died.
"Our goal still stands. We are boycotting today's meeting because the truth has not been uncovered and there has been no breakthrough," Yim Sovann, a lawmaker for the opposition Cambodia National Rescue Party (CNRP) said.
"This meeting is a violation of the constitution."
The National Election Committee says Mr Hun Sen's Cambodian People's Party won 68 seats in the July 28 election to the CNRP's 55.
That was already a big setback for Mr Hun Sen, but the CNRP claimed victory, saying it was cheated out of 2.3 million votes.
The opposition and some political analysts say a quorum of 120 lawmakers is needed to open parliament but Mr Hun Sen has ignored that statement.