Sixteen-year-old Sara McKoy, who organised the mammoth Brisbane march, could well have been auditioning for a political future as she addressed the crowd.

And, judging by the response, it could be a successful path should she choose to take it.

Thousands chanted her name as she took to the stage to deliver the closing address of the day.

“As a society we have failed in our duty to protect this earth, that is why it is so important that we are here today,” Ms McKoy told the crowd.

“Generations before us have refused to acknowledge this climate crisis and it is vital we start.

“I have a message for our prime minister. Scott Morrison, do you understand how serious this emergency is? If you say how much this is a real and important issue then what are you doing to fix it?”

Students, some as young as eight, voiced their opinions to the crowd, which at its peak stretched for about a kilometre down George Street.

“There is no place I don’t see traces of ignorant human behaviour,” eight-year-old Thea Hervall said, without reading from notes.

“Look at how our nation treats its Indigenous people, the people who have answers to many of our problems, the people who have lived in harmony with the environment for thousands of years.”

The gathering began at Queens Gardens park on George Street about 11am and finished with a march to the entrance of Parliament House.

Premier Annastacia Palaszczuk said she did not think students should have wagged school to attend the march, but it was wonderful children were accepting the science of climate change.

"I don't support children leaving school for this rally. I think it could have been held after school hours or it could have been held on a weekend," she said.

Opposition Leader Deb Frecklington agreed.

"Kids can protest in their own time, out of school hours, on weekends, but not during school time," she said.

Traffic was gridlocked around the CBD as motorists abandoned their cars to watch or honked their horns in support.

By 3pm, large groups of activists were heading for home as dark clouds rolled in overhead and threatened to dampen the spirits of those who remained.

Organisers said they were now discussing further strikes if they did not see the action they wanted from politicians.

- with Felicity Caldwell.