HAMBURG (Reuters) - Summits of world leaders are usually highly choreographed affairs that leave little to chance.

French President Emmanuel Macron, German Chancellor Angela Merkel and Dutch Prime Minister Mark Rutte attend the press conference after the meeting at the Chancellery in Berlin, Germany June 29, 2017. REUTERS/Fabrizio Bensch

They are held in locations that are easily shielded from demonstrators. And policy differences are papered over by envoys behind closed doors well ahead of time.

But the G20 meeting in Hamburg next week will be different.

Host German Chancellor Angela Merkel has taken a high-risk gamble by choosing to hold the summit in the center of the northern port city, partly to show the world that big protests are tolerated in a healthy democracy. This has created a huge challenge for police.

On the policy front, she is determined not to surrender too much ground to U.S. President Donald Trump on climate change, trade and migration, possibly setting the scene for an unusual public clash.

“Quite honestly, it is hard to know what will happen in Hamburg,” said a senior German official who has been involved in the preparations and who declined to be identified. “It will not be a summit of great unity, that’s for sure.”

“The biggest concern is security,” the official said. “If we have another Genoa, it will be a failure,” the official added, referring to the 2001 G8 summit in which protesters engaged in violent clashes with police. One person was shot dead and hundreds were injured.

The stakes are high at the July 7-8 summit for Merkel, who is in the midst of a federal election campaign and can ill afford images of chaos and disharmony.

Her relations with three of the most high-profile participants, Trump, Russian President Vladimir Putin and Turkish President Tayyip Erdogan, are strained.

Trump and Putin will be meeting face-to-face for the first time, an encounter which will be intensely scrutinized and could overshadow the entire summit.

Merkel chose Hamburg, the city where she was born before her father moved the family to communist East Germany, to send a message of openness. It was the nightclubs of Hamburg, which traces its history back to mediaeval times and was badly bombed in World War Two, that helped to launch the Beatles.

The city is one of the biggest trading hubs in Europe. It is home to some of Germany’s biggest media groups and perhaps its most potent symbol of left-wing dissent, the Rote Flora, a former theater in the city’s Sternschanze district that has been occupied by anti-capitalist squatters for nearly three decades.

An additional incentive for Merkel, an avid opera fan, was the opportunity to show off Hamburg’s new architectural marvel, the Elbphilharmonie concert hall, where G20 leaders will gather for dinner on the first evening of the summit.

“WELCOME TO HELL”

Ahead of the summit, the German government has played up the fact that protesters will be allowed to voice their opposition to the event. The message to leaders like Trump, Putin and Erdogan is clear: tolerance for public dissent is a cornerstone of a confident, open democracy.

“We are a country where people have a right to demonstrate,” said Merkel’s spokesman Steffen Seibert. “Every citizen has a right to protest. And that counts for the G20 too.”

That message has put huge pressure on German police. If they cannot keep the peace, Hamburg officials have acknowledged, it will amount to a defeat for Merkel in her own backyard.

Some 20,000 police, with dogs, horses and helicopters and 7.8 kilometers (4.8 miles) of steel barriers will be deployed.

Police will be facing off against tens of thousands of protesters who plan to encircle the convention center in the heart of the city where leaders will meet - a far cry from Schloss Elmau, the idyllic, isolated retreat in the Bavarian Alps where Germany held a G7 summit two years ago.

Security officials say that up to 8,000 of the demonstrators will be anarchists and left-wing radicals whose main goal is to disrupt the summit. They are planning a protest on the eve of the G20 that they have dubbed “Welcome to Hell”.

“The policies of the G20 have created hellish conditions in many countries around the world,” Andreas Blechschmidt, one of the organizers of the July 6 protest, told Reuters in front of the Rote Flora, which stands just a few hundred meters from the site where the leaders will meet.

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“We want to show them that we can turn up the heat too,” he said. Behind him, a large banner reading “Capitalism will end anyway, you decide when” hung from the graffiti-covered squat.

Other protests will be held on July 7 and 8.

Police are also worried about clashes between supporters of the Kurdistan Workers Party (PKK), the militant group engaged in an armed struggle against the Turkish state, and Turkish nationalists who support Erdogan.

“More so than Trump, the presence of Erdogan is mobilizing people across Europe,” said Werner Raetz, a veteran German activist involved in the planning of the G20 protests.

CLIMATE, TRADE, MIGRATION

On policy however, the German government views Trump and his “America First” doctrine, as its biggest headache.

At previous G20 summits, a broadly united West led by the United States and Europe would push countries like Russia, China, India and Brazil to follow its lead. But since Trump entered the White House, the Western consensus has broken down.

Trump ignored pleas from his Western allies at a G7 summit in Sicily last month and announced he would pull out of the Paris climate accord. His administration has been threatening countries like Germany and China with punitive trade measures on steel in the run-up to the G20.

Washington even appears to be backtracking on language pledging to “fight all forms of protectionism” that Trump approved at the G7, say German officials, who list migration as another source of contention.

On both climate and trade, Merkel’s best ally in Hamburg may prove to be Chinese President Xi Jinping, who will meet her in Berlin before the summit.

“Everything is open,” said Ulrich Speck, senior research fellow at the Elcano Royal Institute think-tank in Brussels. “There is not a clear Western position. It’s a big change in the international landscape.”

A decade ago, at a G8 summit in Heiligendamm on Germany’s Baltic coast, Merkel managed to coax President George W. Bush - who like Trump had pulled out of a global climate pact shortly after taking office - into supporting a new climate deal with substantial cuts in greenhouse gas emissions.

In the aftermath, German media anointed her “Gipfelkoenigin”, or queen of summits, for her ability to forge sensitive compromises between leaders with disparate views.

But in a speech to the Bundestag lower house of parliament on Thursday, Merkel acknowledged that no such compromises were likely in Hamburg.

“These will not be easy talks,” she said. “The differences are obvious and it would be wrong to pretend they aren’t there. I simply won’t do this.”