BERLIN (AP) — Germany is imposing border controls ahead of the Group of 20 summit in Hamburg next month.

The Interior Ministry says the checks that started Monday will be conducted in a “flexible” manner along Germany’s land, air and sea borders.

Under the rules of Europe’s passport-free travel zone, known as Schengen and of which Germany is a member, border controls can be conducted if security concerns warrant it.

The ministry said in a statement that the measures are designed to prevent people planning violence from entering the country.

Protests are expected for the July 7-8 summit in Hamburg that will see Donald Trump’s first visit to Germany as U.S. president. Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan are also among the G-20 leaders.