A dozen activists gathered around 6:00 am (0400 GMT) at the Dreirosen Bridge in the northern Swiss city to disrupt traffic on the Rhine, through which about a third of Swiss oil imports reportedly pass.
Five of them descended from the bridge and strung a banner demanding "Freedom of movement for people rather than for oil tankers".
Another had a drawing of a stop sign and the word "Colonial".
By mid-morning, Basel police told AFP that around a dozen vessels remained blocked and traffic had been halted "until further notice".
"We are in close contact with the people involved in the operation," a police spokesman told AFP.
"Our top priority is to ensure that none of the people involved are injured," he added.
The activists of the Collective Climate Justice were taking part in a nearby "No Borders" climate camp, which had permission to pitch tents near the bridge.
The Collective published aerial images on the X platform, formerly known as Twitter, showing the activists suspended under the bridge after "over six hours", giving no indication of how long the action would last.
"Quite a lot of supporters on the bridge and in boats. Ice cream is being delivered to the climbers," it said.
The demonstration is the latest in a growing number of disruptive actions by climate activists in Switzerland, including some where participants have glued their hands to the pavement to block traffic on busy highways.
This week, two members of activist group Renovate Switzerland also disrupted the Locarno film festival by gluing themselves to the stage during an awards ceremony, where the organisers allowed them to address the audience with their demands of more climate action.
Such actions have spurred some public anger, with the populist right wing Swiss People's Party -- Switzerland's largest political party -- categorising the non-violent activists as "climate terrorists".
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