The broad OMXS index took a big step up just after 4 p.m., closing up 1.3 percent. The sudden rise coincided with a drop in oil prices.
According to the Bloomberg news agency, it was about President Donald Trump saying that the United States is in the "final stages" of negotiations with Iran.
On the OMXS30 list of the most traded stocks, mining group Boliden took the lead with an increase of 5.1 percent.
Saab's shares rose 2 percent. The defense group has received an order for the Carl-Gustav M4 grenade launcher from Lithuania worth 460 million kronor.
Only a few large-cap stocks were in the red, including pharmaceutical giant AstraZeneca and heat-pump manufacturer Nibe.
A barrel of North Sea oil fell more than 5 percent to just over $105 at the close of trading in Stockholm.
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