Tens of thousands are gathering in Berlin to listen to former U.S. President Barack Obama and German Chancellor Angela Merkel discuss democracy and global responsibility at a Protestant conference as the country marks the 500th anniversary of the Reformation.

German Protestant Church members and visitors from all over the world crowded a square in front of the German capital’s iconic Brandenburg Gate landmark on Thursday amid high security following the deadly attack in Manchester. Up to 100,000 visitors were expected, according to the organizers.

Later on Thursday, the chancellor will also meet current U.S. President Donald Trump when she travels to Brussels to attend the NATO summit there.