German security officials say left-wing extremists have become more willing to use violence over the past five years.

Almost 2,000 more left-wing extremists were considered by authorities to be potentially violent than in 2012.

Their ranks swelled 27 per cent from 7,100 to about 9,000 between 2012 and 2017, domestic intelligence agency BfV said.

German security officials say left-wing extremists have become more willing to use violence over the past five years

A left-wing protester being arrested at a G20 demonstration that devolved into a riot

Police arrest several protesters opposing the G20 summit last July

The number of violent crimes committed by left-wing extremists rose 88 percent during that period, to 1,648 from 876.

Last year saw a spike in left-wing violence partly due to the G20 summit in Hamburg, which led to a series of riots and arson attacks around Germany.

German railways were attacked 13 times by arson in the leadup to the summit, parked cars were burned, and shops looted.

Police arrested 186 people overall and temporarily detained another 225 in the weeks before and days during the G20.

A man is dragged away by riot police at a protest ahead of the G20 summit attended by many left-wing extremists

A shop looted by left wing mobs during riots ahead of the G20 summit in Hamburg

A parked car burns as protesters wreak havoc in riots ahead of the G20 summit

Almost 2,000 more left-wing extremists were considered by authorities to be potentially violent than in 2012

Last year saw a spike in left-wing violence partly due to the G20 summit in Hamburg, which led to a series of riots and arson attacks around Germany

German police estimated far-left protesters committed more than 2,000 crimes in that time, including vandalism, bodily harm, disturbing the peace, arson, and resisting arrest.

The rise of the far-right Alternative for Germany also gave left-wing extremists a new target in recent years.

BfV chief Hans-Georg Maassen called for more public debate about the need to reject violence as a means of achieving political ends.

A looted supermarket during widespread anarchy led by left-wing protesters angry at the G20

An injured protester is taken away by paramedics during violent clashes between police and left-wing extremists last year