The campaign group’s famous ship is carrying youth strikers from communities most hit by climate change to demand world leaders “stop failing us”.
Greenpeace said it had been warned by port authorities not to sail up the River Clyde to the global climate conference, but added the vessel would still attempt the journey.
If the voyage is successful, the four youth activists on the Rainbow Warrior plan to meet fellow members of the Fridays for Future climate strike movement on Monday afternoon outside the summit to deliver their message.
They are warning that the climate talks should not go ahead without people who are most affected, but say many activists have been shut out by a failure to distribute vaccines equally between countries and travel restrictions, while major nations have big delegations attending.
The Rainbow Warrior set sail from Liverpool on Saturday night, and contacted the Clyde port authority to request permission to berth outside the Cop26 conference, but was told it could not sail up the Clyde and the area was controlled by police.
The captain decided to ignore the warnings and will continue the ship’s journey as the activists’ message and presence at Cop26 is fundamental to its success, Greenpeace said.
A “stop failing us” message is written on large banners hung between the Rainbow Warrior’s masts and bows.
Speaking onboard, 19-year-old climate activist Maria Reyes, from Mexico, said: “From vaccines to visas and travel restrictions, we’ve already had to overcome many obstacles that the Cop26 organisers tried to use in an attempt to shut us out.
“But we’re here, we’re coming and we won’t be stopped.
“Inequalities such as gender violence, racial discrimination, class inequality and forced migration are exacerbated by the climate crisis.
“By denying us entry these so-called ‘leaders’ are fanning the flames of these inequalities. Enough empty speeches, there won’t be climate action without climate justice.”
Edwin Namakanga, 27, from Uganda, said: “World leaders should be rolling out the red carpet to people most affected by this crisis, not denying us from making our way to Cop26.
“We’re only four activists but we’re representing millions and our voices must be heard.”
Police Scotland Assistant Chief Constable Gary Ritchie reiterated that no vessels are allowed in the prohibited security zone until the end of the conference.
He said: “The river is subject to movement and mooring restrictions for security purposes. No vessels are allowed in the prohibited security zone until the event ends on 12 November.
"River users have a responsibility to check the Notice to Mariners prior to departure so they're aware of what bodies of water are accessible and which are restricted.”
He added: “The Ministry of Defence Police is on the river at all times during the event in support of Police Scotland.”
Most MDP officers are armed with the Heckler and KochMP7 semi-automatic carbines, and a semi-automatic pistol. Depending on role, others may be armed with a SA80A2 rifle, Colt Canada C8 rifle, L129A1 (LM7) rifle, Sig Sauer P229 pistol, X26 Taser and H&K AEP Launcher.