Thousands have attended a rally in the northern German city of Hamburg to protest at an imminent meeting of leaders of the world's major industrialized and emerging economies, known as G20.

Police said nearly 10,000 people gathered outside the city hall in Hamburg while canoeists and Greenpeace activists held similar rallies on the nearby river Alster and in the city's port.

Organizers said the figure was higher than the one put by police. They hope a total of 100,000 could attend similar protests that are planned for the days leading to the G20 summit on Friday and Saturday.

A police spokesman said the main protest in Hamburg was completely peaceful. However, officials fear upcoming rallies could turn violent as the bustling Hamburg, a main bastion of extreme, anti-establishment left-wing groups, could become the scene of possible clashes between police and protesters. Interior Minister Thomas de Maiziere warned about any violence that could lead to property damage.

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Performers wearing masks of world leaders hold a placard reading: Better off equal! Close the gap between rich and poor during a demonstration called by several NGOs ahead of the G20 summit in Hamburg on July 2, 2017.

Freedom of assembly is only valid for peaceful demonstrations, he said in the Sunday edition of the daily Bild, adding that any violence should be nipped in the bud.

Security will be tight around the venues of the G20 summit on Friday and Saturday as police is expected to deploy around 15,000 police officers to protect the event. Some 3,800 officers will also monitor airport and train security.

The G20 summit is where US President Donald Trump will make his first visit to Germany. That could make the situation worse as many have vowed to take to the streets to protest against controversial decisions he has taken over the past months, including a withdrawal from the Paris climate agreement and imposing a ban on the entry of nationals from six Muslim-majority nations to the US.