Meanwhile, his supporters have praised what they see as his commitment to protecting U.S. jobs.
A fractious first North Atlantic Treaty Organisation summit with Trump also left European allies wondering where the military alliance goes next.
Hamburg police chief said that action will not be limited to "just sit-in protests, but massive assaults", The Guardian reported.
At a press conference in Berlin Monday for the launch of her party's manifesto, Merkel declared that there remain "a whole series of thorny issues" within in the G20.
According to a German publication, a total of 30 demonstrations have been announced before the end of the summit.
"While we seek chances to cooperate for everyone's benefit, globalization is seen in the American administration as a process which isn't about win-win situations, but about winners and losers", she said.
Without mentioning the protests that have German security officials anxious about possible acts of sabotage this week in the country's second-largest city, Merkel noted that these non-traditional issues were forced onto the G20 agenda.
The BRICS leaders' informal meet will at least ensure the two leaders meet away from the formal G20 summit and give them the opportunity to directly speak to each other.
Officials have said as many as 8,000 demonstrators, including many intent on violence, are likely to converge on the city for the two-day summit.
Sunday's demonstration was organised by a group called " Protest Wave G20".
U.S. President Donald Trump and Russian President Vladimir Putin will also meet at the summit, and Germany will try to push climate change and free trade to the top of the agenda.
Dirk Messner, director of the Centre for Advanced Studies on Global Cooperation Research said "our country-first ideologies" pursued by some countries are unsafe since we live in the times of global interdependency.
Hamburg is boosting its police force with reinforcements from around the country and will have 20,000 officers on hand to patrol the city's streets, skies and waterways.