The British Supreme Court on Monday morning started hearing the historic Brexit legal case to decide whether the British government can begin the process to pull Britain out of the European Union (EU) without parliament's approval.
Protesters from both the leave and remain camps gathered at Parliament Square to express their feelings ahead of the case at the Supreme Court, which is scheduled to last four days.
The government has insisted it has the power and authority to start the exit process by triggering Article 50 of the Lisbon Treaty, the formal gateway for a member country to leave the EU.
British Prime Minister Theresa May wants to trigger Article 50 by the end of March. However, the High Court heard a challenge to May's plans and ruled that she must get the parliament's support to invoke the clause.
May's government appealed against that ruling and the 11 Supreme Court judges are expected to make a definitive decision in the coming days. If the judges uphold the High Court decision, it will mean May must involve parliament in her Brexit plans. May's supporters fear that politicians could use that process to delay or even sabotage Britain's exit from the EU, even though the majority of people voted to leave in the June 23 referendum.
On the other hand if the judges, when they announce their decision in January, agree with the government, it will pave the way for May to start the process without seeking the approval of parliament.
Commentators say that what is really on trial in the hearing is democracy and the authority of the government.
The judges were not in their traditional ceremonial robes for the hearing, though it is the first time in history all law judges have sat together.
That is an indication of the constitutional and legal importance of the case.
The somber atmosphere of the wood paneled courtroom is in sharp contrast to the noisy demonstrations outside the historic building close to the Houses of Parliament.
The first full day of the hearing will be taken up with the government's Attorney General Jeremy Wright contending that the government, using centuries old powers, has the right to start the process. He also insists that parliament legally approved the referendum handing the decision to the people of Britain.
Later in the week, the judges will hear from the lawyers representing Gina Miller, a wealthy businesswoman who originally took the case to the High Court to force May to hand the decision of triggering Article 50 to parliament.