"This is the beginning of change," said university student Ahmed al-Mokayed, attending with his brother and cousin.
Businessman Abdul Mohsen al-Mani, who brought his two sons to the film, was ecstatic, after being denied public cinema for some three decades.
"This is the first step in a peaceful revolution," he told AFP.
"I don't want my two sons to grow up in the dark ... I told them that in the future they will talk about today like a joke," he added.
It was long in coming -- and no one is certain that it will launch a thriving public cinema industry, with strident opposition from clerics who regard film, music and other entertainment as violating Islamic teachings.
Police at the venue had to fend off a small band of conservative Muslims who warned that films were bringing disasters on the country, citing a recent series of minor earthquakes in western Saudi Arabia.
"Allah is punishing us for the cinema," one said. "It is against Islam."
"Menahi", a comedy about a Saudi country bumpkin getting lost in the big city, was shown in December to huge crowds in the relatively free-wheeling Red Sea city of Jeddah.
Rotana group, the Saudi-owned regional entertainment giant, had hoped to follow that up quickly with Riyadh showings.
But it has taken five months to get government permission, admitted Ibrahim Badei, a Rotana public affairs official.
In between, attempts to play the film in others cities were blocked by religious authorities, raising the stakes for its debut in the extremely conservative capital.
To avoid that risk, Rotana only announced the Riyadh showing on Friday and most people learned of it from newspaper stories the following day.
Badei called it a "surprise opening."
A phalanx of police was recruited to fend off the Islamic activists, whose association was unknown.
Those attending the film said it gave hope to the growing Saudi reform movement fighting the power of conservative clerics.
"Our society is being kidnapped by these people," said businessman Mani.
The fight is being led in part by Rotana's main owner, flamboyant billionaire Prince Alwaleed bin Talal, a nephew of Saudi King Abdullah. "Films and movie theatres will come inevitably," he said in February.
Badei said that Rotana plans to show Menahi at least three more times over the coming weeks, and expects crowds to be larger and include women.
In Jeddah's showings, the sexes were separated, with men on the ground floor and women in the balcony.
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