Italy's Prime Minister Mario Draghi has called on world leaders to uphold multilateralism and do all they can to overcome differences.

Speaking at the opening of the G20 Summit, Draghi, whose country holds the G20 presidency this year, reminded people that the world faced protectionism, unilateralism and nationalism even before the pandemic.

"But the more we go with all our challenges, the more it is clear that multilateralism is the best answer to the problems we face today," he said in the Rome Convention Center, also known as La Nuvola, or The Cloud. "In many ways, it's the only possible answer."

Draghi said that going alone is not an option to tackle challenges from the pandemic to climate change.

"We must do all we can to overcome our differences. We must rekindle the spirit that led to the creation of this group."

He warned of the "startling disparity in the global distribution of vaccines", citing the fact that in high-income countries 70 percent of the population has received at least one dose, while in the poorest countries the percentage is just 3 percent.

"The differences are morally unacceptable, and undermines the global recovery," said Draghi, a former governor of the European Central Bank.

His words were echoed by United Nations Secretary-General Antonio Guterres, who urged leaders at the G20 Summit to show leadership to save lives, prevent further suffering and enable a full global recovery.

"Vaccine inequality is prolonging the pandemic at great human and economic cost," he said.

According to a recent report by an activist group including organizations such as Oxfam, Amnesty International and UNAIDS, wealthy countries promised to donate 1.8 billion doses of COVID-19 vaccines but have delivered just 261 million, or 14 percent, of the pledge.

At the meeting on Saturday the leaders of the world's 20 major economies endorsed a 15 percent global minimum corporate tax. Their commitment came after about 130 countries agreed in early October to a 15-percent minimum tax rate.

The minimum tax deal, which was expected to be adopted on Sunday, fell short of the original call by the United States for a 21 percent minimum tax.

US President Joe Biden, who is desperate to claim a win after his infrastructure bill was stalled in the US Congress and whose approval rating has plummeted, said on Saturday that "this is more than just a tax deal－it's diplomacy reshaping our global economy and delivering for our people".

As leaders started their first day of the two-day meeting, thousands of climate activists and anti-G20 campaigners marched through the streets of Rome, accusing leaders of the world's major economies of inaction on climate change.

The protests, in which participants included movements such as Friday for Future and Extinction Rebellion, came just a day before the 2021 UN climate conference which opened in Glasgow on Sunday.

Allison Pearson, a columnist for the British newspaper The Daily Telegraph, said in a tweet: "G20 leaders fly to Rome. Carbon emissions? …Then fly to Glasgow. Carbon emissions? Disgusting hypocrisy."

The Italian government has deployed more than 5,000 extra police officers in addition to the usual police force in Rome to ensure security during the two-day meeting. It has also designated a 10-square-kilometer maximum security zone surrounding the meeting venues.

On Saturday armed soldiers and police officers were seen not just around meeting venues but outside many luxury hotels in the city center where G20 leaders will be staying. Some patrol helicopters buzzed overhead.