Extinction Rebellion Scotland's vessel was placed in Trongate on Monday, covering the intersection of Gallowgate and High Street, by the Merchant City clock tower.
The boat carries a message from the group which says Act Now, understood to be a demand for the UK Government to do more to tackle the climate crisis.
A message saying "The future you fear is already here" is on the other side of the vessel.
Scores of campaigners attended the protest, surrounded by a heavy police presence.
Elliot Blaauw, 66, of Inverness, was one of the protesters ready to risk arrest by gluing themselves to the boat if necessary.
He said: "Waters are rising, we are rising too. We're all going to take to the boats.
"The Clyde will overtop in 20 years, so that's what the boat is for, it's a symbol of the water is rising."
He criticised the UK and Scottish Governments for declaring a climate emergency then opening up new areas for drilling for oil.
He said: "They're vowing to go for every last drop of oil.
"Scientists tell us that the oil we have now is enough to send the temperatures to two, three degrees."
He added: "I'm a grandparent, my interests are my children and my grandchildren. Drilling for more oil - which bit of climate emergency don't they get?"
Mr Blaauw said people had been supportive of the protest, adding: "This is act now - they say they care about climate change, they say they've heard us, they say they understand the scientists.
"Scientists say if we keep on like this we're going to two, three, four degrees.
"We must act now, we're running out of time. Two more years and our climate budget is blown."
Glasgow City Council said the Trongate was closed to all eastbound traffic between Albion Street and High Street due to the continuing protest.
The local authority urged road users to consider taking other routes and warned there could be some congestion on surrounding streets.
Extinction Rebellion protests were also to take place in Bristol, Cardiff, Leeds and London.
It comes as part of the group's week-long "summer uprising".