• Mr. Trump’s trip to Europe is expected to culminate with a high-stakes meeting with President Vladimir V. Putin of Russia on Friday.

‘Welcome to Hell’ protesters vow to disrupt G-20 meeting.

In what may be a preview of the scene in Hamburg during the meetings between the leaders of the world’s 20 leading economies, protesters converged Thursday night for a demonstration called “Welcome to Hell.” Up to 100,000 protesters were expected in the coming days, and 20,000 police officers from across Germany and neighboring countries were dispatched to provide security.

The crowds massed on Thursday just beyond the area cordoned off by the police where the summit meetings were scheduled to take place. On Wednesday night, several peaceful protesters filled the streets, gyrating to techno dance beats. But the crowds on Thursday were larger and more aggressive.

Protesters clad in black and wearing balaclavas, which are forbidden by law during protests in Germany, threw bottles and stones at helmeted police officers who responded with water cannons and pepper spray.

Residents and workers at businesses in the area were anxious.

“We’re a bit worried that the protesters will destroy the shop, and I’m worried about being able to get home,” said Jennifer Gand, who works at a retail kiosk in the St. Pauli district in central Hamburg. “But what can you do other than treat the protests like they don’t matter and carry on?”