Police used fire, tear gas and water cannons to break up the crowds in Brussels and Rotterdam, and almost 100 individuals were detained.
On Saturday, Germany recorded its highest daily number of infections in a single day.
A manhunt for 46-year-old Jürgen Conings began Tuesday after the suspect made several threats against a prominent virologist and armed himself with a rocket launcher, a machine gun and an ordinary pistol.
The festival is being advertised on social media as a sequel to an April Fools' Day party that drew thousands of people to one of Brussels' largest parks, Bois de la Cambre, and ended in clashes with police earlier this month.
Belgian Prime Minister Alexander De Croo said he understands festival-goers' pandemic fatigue, but "being in solidarity today is the key to our freedom tomorrow."
A festival titled La Boum ("The Party") drew a reported 2,000 people to a Brussels park, although police warned it was a fake event set for April Fools' Day.
The 15th-century artwork had adorned the wall of the Town Planning and Development Counselor's office for about 60 years until experts began asking questions about its authenticity.
The private party at a house in Belgium was shut down on Friday after police were called to the scene around 9:30 p.m.
The latest rallies join a string of anti-mask and anti-lockdown protests reported this month in Europe, including in the U.K. and Germany.
United States Secretary of State Mike Pompeo is touting an international coalition against China, using his social media accounts to highlight criticisms of the Communist Chinese government from across the globe.
"One sophomoric response does not deserve another," Graham said of Trump's rebuke to Pelosi.
"For over five years, the United States has pursued diplomatic and technical avenues with Russia aimed at preserving the INF Treaty," NATO Secretary-General Jens Stoltenberg said.
On Friday, Belgium's brewers will meet to honor Saint Arnould, the patron saint of brewers.
Given President Donald Trump's animosity toward both John McCain and NATO, such a move could be seen as a challenge to the White House.
The president made the claim in an unscheduled press conference Thursday and said he told allies, "I would be very unhappy if they didn't up their commitment very substantially."
The letters said America's contribution to the NATO alliance is "no longer sustainable."
Three people have been charged with involvement in the planned attacks.
The Belgian Foreign Ministry had to intervene to get some of the men into the U.S.
The EU's security landscape is worlds beyond what it was just two years ago.
We could see women play a greater role in attack plots in the West, writes Devorah Margolin.
With May's cabinet in open disagreement, those who want Britain to stay in the EU are feeling optimistic.
Belgian intelligence believes the network may have stored explosives at a garage on a school's premises.
Will the president use the Polish visit to pick a fight with Berlin, Paris and Brussels?