Thailand's Constitutional Court has dismissed an opposition complaint against the government's proposed constitutional changes.
"The court dismiss all the petitions," said Nurak Marpraneet, one of eight judges at the Bangkok court, which had been surrounded by security forces in anticipation of the ruling.
The court found that complaints against plans by Prime Minister Yingluck Shinawatra's party to amend the constitution -- drawn up under the military junta that deposed her divisive brother Thaksin -- were unfounded.
The opposition Democrats, which were swept out of power in a landslide election win for Yingluck last year, alleged that the efforts to amend the constitution signalled a threat to the deeply-revered monarchy.
"There are not sufficient facts to convince the court that the defendants are trying to overthrow the constitutional monarchy," said Nurak, adding that a drafting assembly had yet to be elected.
The court said a referendum was necessary to decide whether the government could go ahead with the changes.
It added that the changes did not threaten the role of the monarchy, as some opponents had argued.
Hundreds of police surrounded Thailand's Constitutional Court on Friday as the country braced for the ruling.
Several anti-Thaksin protesters gathered outside the court, while a faction of the rival "Red Shirts" - mainly rural, working class supporters of the fugitive former premier - also held a small rally near parliament in preparation of a larger protest if the court had blocked change.
National police spokesman Major General Piya Utayo said around 600 officers were deployed in and around the court, with further hundreds more on standby nearby.
Two pro-Thaksin premiers were forced from office in 2008 in judicial rulings, making way for the Democrats, who are backed by the military and Bangkok elite, to take power in a parliamentary vote.
Yingluck's Puea Thai party swept to power last year on a wave of Thaksin support following deadly 2010 Red Shirt street protests, promising to amend the constitution that was drawn up in 2007.
The party is trying to amend the section of Thailand's charter governing the process for changing the constitution in order to allow it to set up a redrafting committee and to put their recommendations to a referendum.
But the Democrats, who also fear the move could be used to open the way for Thaksin's return, allege the underlying motivation threatens Thailand's system of constitutional monarchy.