HAMBURG Protesters threw bottles and torched cars, injuring seven police officers, before a G20 summit in Hamburg on Thursday, tarnishing the start of a meeting Chancellor Angela Merkel hopes will cement her role as a stateswoman as she seeks re-election in September.

Merkel, who is campaigning for a fourth term, can ill afford images of chaos and disharmony. The summit, which starts in earnest on Friday, is a chance for her to polish her diplomatic credentials but would be disastrous if marred by widespread violence.

She met U.S. President Donald Trump for an hour on Thursday evening, but less than an hour later police clashed with anti-capitalist demonstrators near the summit venue and fired water cannon at black-clad protesters after they threw bottles.

A Reuters eyewitness saw at least one protester with blood on his face being treated. “Welcome to Hell” was the protesters’ greeting for Trump and other world leaders arriving for the two-day meeting. Police said at least seven officers were injured in clashes that continued after a late sunset in the northern city.

Merkel has taken a high-risk gamble by choosing to hold the summit in the northern port city of Hamburg, partly to show the world that big protests are tolerated in a healthy democracy.

Before meeting Trump, she struck a consensual tone, holding out hope for agreement on the divisive issue of climate policy and pledging to broker compromises. She pledged to represent German and European interests at the summit, but added:

“On the other hand, as hosts we – and I – will do all we can to find compromises.”

Trump faces a testy confrontation at the summit with leaders of the other big Group of 20 economies after deciding last month to pull the United States out of the 2015 Paris climate deal.

German Foreign Minister Sigmar Gabriel cited “many commonalities” on foreign policy after a meeting that included Merkel, Trump, U.S. Secretary of State Rex Tillerson, and Trump family members and advisers Jared Kushner and Ivanka Trump.

But he told German broadcaster that “clear differences” on climate change and trade continued to divide the two allies.

Merkel has held out hope that an agreement could yet be found on climate.

“There are various options, which can be discussed. We know that the United States have withdrawn. All others … or as far as I know, many many others stand by this agreement,” she said.

As the leaders began holding informal meetings, thousands of protesters from around Europe, who say the G20 has failed to solve many of the issues threatening world peace, poured into Hamburg to join the main demonstration.

Police expected around 100,000 protesters in the port city, some 8,000 of whom are deemed by security forces to be ready to commit violence. At least 13,000 protesters joined the main march on Thursday, including around 1,000 black-clad anarchists, police said. Up to 20,000 police officers from across Germany are on hand.

DELICATE BALANCE

As summit host, Merkel must seek consensus among the G20 leaders not only on the divisive issue of climate policy but also on trade – an area fraught with risk as Trump pursues his ‘America First’ agenda.

Indonesian finance minister Sri Mulyani Indrawati said Merkel must be careful not to allow acrimony to undermine the summit.

“There is quite a delicate balance that Angela Merkel will have to navigate in a way, because it is not clear that being confrontational won’t just create even more of a credibility problem for G20 cooperation,” she told Reuters.

Merkel earlier said she was committed to an open international trading system, despite fears of U.S. protectionism under the Trump administration.

“We’re united in our will to strengthen multilateral relations at the G20 summit … We need an open society, especially open trade flows,” Merkel said in Berlin.

She and Trump discussed G20 themes, North Korea, the Middle East, and the conflict in eastern Ukraine, a German government spokesman said. Turkey’s Tayyip Erdogan is among other leaders Merkel was to meet.

Trump, who earlier in Poland called again on NATO partners to spend more on defense and said he would confront the threat from North Korea, will also hold his first face-to-face meeting with Russian President Vladimir Putin on the sidelines of the summit.

Their meeting, scheduled for Friday, will be closely watched at a time when mutual ties remain strained by U.S. allegations of Russian election hacking, Syria, Ukraine and a U.S. row over Trump associates’ links to Moscow.

Ahead of the meeting, Putin threw his weight behind the Paris accord.

“We see the Paris Agreement as a secure basis for long-term climate regulation founded on international law and we want to make a comprehensive contribution to its implementation,” he told German business daily Handelsblatt.

(Additional reporting by Thomas Escritt, Roberta Rampton, Noah Barkin, Andrea Shalal, Emma Thomasson; Writing by Paul Carrel; Editing by Richard Balmforth)