Anti-monarchists are expected to stage large-scale protests in London on Saturday, despite warnings from authorities and a controversial new law.

More than a thousand people will be protesting in Trafalgar Square, according to Republic, a London-based campaign group advocating to replace the British monarchy with an elected head of state.

Meanwhile, London's Metropolitan Police Service said it will have more than 11,500 officers on duty Saturday, making it "one of the most significant and largest security operations" that the agency has led.

Under a new law that went into effect this week, protesters who block roads and railways could face up to 12 months behind bars and a fine, and police will be empowered to stop and search protesters suspected of having intent to commit an offense.

The coronation festivities kicked off Friday with a welcome reception for foreign guests hosted by King Charles III at Buckingham Palace.

U.S. First Lady Dr. Jill Biden and Ukrainian first lady Olena Zelenska were both spotted entering the palace for the reception.

Inside the palace, Biden and her granddaughter Finnegan Biden were photographed speaking with Kate, the Princess of Wales.

Charles was photographed in conversation with Zelenska.

Around 100 heads of state and 20 royal families will attend Saturday’s coronation service.

In addition to Charles and Kate, other royals at Friday's reception included Prince William; Princess Anne; and Prince Edward and Sophie, the Duchess of Edinburgh.

Royal fans camped out on The Mall ahead of Saturday’s coronation got a royal surprise Friday when they were greeted by King Charles III, Prince William and Kate, the Princess of Wales.

The trio stepped out of their cars and did a walkabout, taking selfies with and shaking the hands of fans who lined the path one day ahead of the coronation service.

Charles and Camilla, the Queen Consort, will travel the same path tomorrow in a procession that will take them to and from Buckingham Palace and Westminster Abbey.

Coronation buzz is spreading across London as the city prepares for the once-in-a-generation event.

Tens of thousands of people -- including the 2,000 people attending the coronation service itself -- are expected to stream into the city for the celebration.

For the past several weeks, workers have been seen cleaning Buckingham Palace and other landmarks, building grandstand seating areas for viewers and hanging flags across the city. In the darkness early Wednesday, coronation participants held a full-dress rehearsal for the big day.

Security is also tight all around London as officials brace for expected anti-monarchy protests.

As the new king and queen passed by in a golden carriage, hundreds of anti-monarchy protesters booed loudly in Trafalgar Square.

They also yelled chants of "what a waste of money" and "not my king."

-ABC News' Patrick Reevell