Four members of staff at the Gallery were tied up and a pistol was loaded in front of them as two thieves took 20 pictures and panels from the walls and display cases.
The masked raiders, who were also armed with a sawn-off shotgun, escaped before the alarm was raised. Police now fear that the stolen pieces could be taken abroad.
Richard Green, who has been curator at the gallery for more than 20 years, said: "The two men tied up and threatened four members of staff as they prepared to close for the evening. Around 20 pictures and panels were taken. Some were cut out of their frames and put into holdalls they had with them. We are still assessing the damage but the value of what was taken is around pounds 1m.
"A Turner watercolour from the 1820s was taken after they smashed the display case it was in. To have 20 works taken leaves a huge dent in our collection. It is a very black day. But mercifully none of the staff were hurt although they are very, very shaken by the ordeal."
Detective Inspector Phil Metcalf of North Yorkshire Police said: "These men knew what they were looking for. They took mainly small paintings and had black holdalls to put them in.
"In cases like this there is always a danger that the works will be taken out of the country and we have alerted the relevant authorities.
"Forensic officers are now at the scene and we are appealing to anyone who saw anything suspicious in or outside the gallery to come forward."Reuse content