"serious crowd" is correct and usable in written English.
You can use it to refer to a group of people whose behavior or demeanor is serious or somber. For example, "The judge's decision was met with a serious crowd in the courtroom."
As far as I remember, that was the first time John generated any serious crowd problems.
It was also the kind of thing worth braving serious crowd nuisances for: an actual collective ah.
Then she laughs at the death of her enemy, the psychotic Joffrey, giggling amid a serious crowd.
The serious crowd included Amy Ryan in a flat (actually navy) Calvin Klein toga and Jennifer Garner, in embroidered silk taffeta from Oscar de la Renta.
He has also replaced John Scanlon, the chief of patrol, who was in command during two of the department's most serious crowd control failures in recent years.
Fears have been heightened by the serious crowd trouble at the recent French cup final, played between old rivals Paris Saint-Germain and Olympique de Marseille at the Stade de France.
So the trick is to ensure that serious crowding is avoided in the first place.
Curtis has a reputation as a top public high school, despite serious crowding.
It is almost certain that serious crowding will be a feature of the transport infrastructure during the games.
Despite their excellent home form, the only serious crowds that turned up were at the derby fixtures, or when Alessandro Del Piero came to town.
Bournemouth is following the example of Narrowneck Beach, Australia, and Mount Maunganui in New Zealand by constructing Europe's first artificial reef; online surf forums talk of "serious barrels", although the hype around it may also lead to some serious crowds.
