The Supreme Court began hearing arguments Monday on President Barack Obama's divisive health care overhaul, a landmark case that could affect the lives of nearly every American.

Supporters of the measure say the provision is needed to spread the cost of health care among all Americans. Opponents are arguing the measure violates the Constitution and tramples on individual liberties.

The court is hearing a challenge by attorneys representing private parties and 26 states. Their main argument is against a provision in the law requiring nearly all Americans to buy health insurance. Many businesses must also offer insurance or pay a penalty.

The court's nine justices will hear six hours of oral arguments in the course of three days, the most the court has scheduled for a single issue since the 1960s.

As the arguments began Monday, scores of protesters in favor of the legislation waved signs, sang and chanted “Support Obamacare” outside the Supreme Court. Fewer against the measure gathered, but some of who those did traveled far to be there with one man saying he came from California. VOA reporters at the courthouse said the two sides took part in impromptu debates, but that the scene remained peaceful.

Inside the nations' highest court, the first session is focusing on the technical question of whether a challenge to the law can come before the court before it has been fully implemented. Mr. Obama signed the law in 2010, but key portions do not begin until 2014.

The court is expected to issue its decision in June.

The law — derisively labeled “Obamacare” — seeks to extend medical insurance to millions of Americans who do not have any. It has become a rallying point for conservatives who claim the changes will lead to bureaucrats replacing doctors in medical decision-making, and that the quality of health care will diminish.

Opponents of the requirement to buy health insurance, called the individual mandate, say Congress lacks the authority to force Americans to purchase the coverage. In briefs filed to the court, they say the mandate is “unprecedented” and a decision upholding it would do “irreparable damage” to the constitutional system.

The Obama administration has argued Congress does have the authority under its Constitutional powers to regulate interstate commerce and levy taxes.

The health care law also bars insurance companies from denying coverage to people with pre-existing conditions or placing a cap on the benefits available to those with serious medical conditions.

Proponents of the individual mandate say it will expand the market of healthy people who otherwise would not purchase insurance, helping offset the cost of covering people with higher medical bills.

This case comes before a divided bench made up of five justices appointed by Republican presidents and the rest appointed by Democrats.

The health care overhaul was the most significant reform to the U.S. health care system in four decades and a key portion of Mr. Obama's domestic agenda.