German President Frank-Walter Steinmeier called on citizens to defend freedom and democracy during a ceremony marking the entry into force of the country's constitution and the founding of the modern federal republic 75 years ago.
"Our democracy is a resilient one," Steinmeier said during the speech delivered in Berlin's government district.
"I am certain of one thing: we are living in testing times. Rough, even tougher years lie ahead of us. The answer to this cannot and must not be faint-heartedness or self-doubt."
Steinmeier warned of threats to democracy, such as the threat posed by Russia.
No one knows when Russian President Vladimir Putin's hunger for power will be satisfied, the president said. "We must do more for our security. We must invest in our defence. We need to strengthen our alliance. And we need the financial means to do so."
He also mentioned climate change, social security and the economic crisis as key issues that could trigger new conflicts in society.
"We will not have fewer disputes in the coming years, perhaps more. The battle for financial resources will become tougher, and with it, of course, the dispute over what is important," he said.
Steinmeier called on the democratic parties to work together where the common good is affected or threatened. "The unity of democrats is needed when democracy is challenged."
The German constitution, or Basic Law, came into force on May 23, 1949, which is also the founding date of the Federal Republic of Germany. The Peaceful Revolution in the former East Germany, which marks its 35th anniversary this year, is being commemorated on Thursday.
The Basic Law initially only applied to West Germany, and only after reunification did it become the constitution for the whole of Germany.
Longtime former chancellor Angela Merkel called for a commitment to democracy said "we must protect" the Basic Law.
She said on the sidelines of the celebrations that she was determined to fight to ensure that future generations still have "the good fortune to have the Basic Law."
Merkel described violence against politicians as a major problem, particularly in local communities. People are involved there who have less protection than top politicians. "That's why we have to defend ourselves together," she said.
During his speech on Thursday, Steimeier said the Basic Law was a "great gift" for Germany after the tyranny of the Nazi regime.
"A gift that must not only be remembered, but that we must cherish, preserve and defend in the everyday life of the republic," he said.
It is one of the oldest constitutions in the world and has become a model for many constitutions, the president said.
The ceremony was attended by the heads of Germany's five constitutional bodies. In addition to the president, these included the presidents of the lower and upper house of parliament, the Federal Constitutional Court as well as Chancellor Olaf Scholz.
A total of about 1,000 police officers were deployed in and around the government district of Berlin on Thursday, a spokeswoman said, including on patrol boats on the Spree river that runs by the main government buildings.
The situation was calm and relaxed in the morning, police said. A total of 1,100 guests were expected on the square in front of the Chancellery.
The ceremony was preceded by an ecumenical service with the participation of several religions in Berlin's St Mary's Church.
After Thursday's official ceremony, citizens will also have the opportunity to celebrate their constitution during a Democracy Festival in Berlin's government district from Friday to Sunday, and on Saturday in the old government district of Bonn, where Steinmeier plans to open to the public his official residence, the Villa Hammerschmidt.
