The leaders - 35 men and two women - posed for the "family photograph" traditionally taken at G20 gatherings, before heading to the plenary chamber room for initial round-table discussions, to be followed by a series of one-to-one conversations between leaders in the center's maze of meeting rooms. "We are debating (trade and climate change) more closely because we want to reach the consensus of all the countries involved".
"Given the role that Turkey has played in this, given that the murder happened at the Saudi consulate in Istanbul, this will be an interesting meeting", said Willis Sparks, director of global macro politics at Eurasia Group.
British Prime Minister Theresa May's attendance at the summit marked the first time a United Kingdom prime minister has visited Argentina's capital.
While the US, Canada and Mexico held a signing ceremony of their new trade pact on the sidelines of G20 summit, China and France held a joint press conference to demonstrate their commitment to reducing climate change.
Joining other forecasters, economists at the Organization for Economic Co-operation and Development last week downgraded their outlook for global economic growth next year to 3.5 percent from a previous 3.7 percent.
After initial plans for him to stay away from the summit, Trump's hardline trade adviser, Peter Navarro, was added to the USA delegation at the last minute and is expected to attend the meeting between Trump and Xi, a U.S. official and a source familiar with the situation told Reuters.
Trump on Thursday cancelled a planned bilateral with Putin over Russia's naval skirmish with Ukraine in waters near Moscow-annexed Crimea.
Officials familiar with the meeting's planning said Trump himself determined Navarro, who flew to Argentina about Air Force One on Thursday, should attend.
Jan Vertonghen set to make Tottenham comeback in Champions League
But of course it will be so hard and we need to focus now on Sunday - another derby against Arsenal . I expect they'll play with the best players but today I don't know how they'll prepare the game.
Xi and other leaders from the BRICS group of leading emerging economies - Brazil, Russia, India, China and South Africa - issued a statement calling for open worldwide trade and a strengthening of the World Trade Organization (WTO).
Eight minutes before he was set to meet with the summit's host on Friday morning, Trump began tweeting about the Cohen saga, saying his business dealings while a candidate were "very legal & very cool". The Kremlin said it was willing to be "patient" in arranging a meeting with Trump.
Inside the talks, uncertainty prevailed about how Trump, known for his unpredictability, would behave at what was shaping up as one of the group's most consequential summits.
British Prime Minister Theresa May told the prince that the killers of Khashoggi should be held to account, her office said after the two leaders met. Saudi Arabia said the prince had no prior knowledge of the murder.
Speaking to reporters at the White House, he said that while China was interested in striking a deal, "I don't know if I want to do it" and "I like the deal we have now".
The speech marked the latest effort by Xi to paint China as a defender of the global trade system at a time when Washington is accusing the country of engaging in unfair trade practices, stealing trade secrets, and subsidising state-owned enterprises at the cost of other countries.
Russian President Vladimir Putin and Saudi Crown Prince Mohammad Bin Salman high-fived each other at the beginning of the G20 forum. "So I really don't know", the President said before departing Washington. Putin reportedly pulled out a pen and paper to sketch the skirmish in the meeting, Bloomberg reported.
A Kremlin spokesman said Putin was ready to continue talks with Trump.
Oil markets will also be closely watching a bilateral meeting between Putin and Saudi Prince Mohammed on Saturday afternoon for any sign of a breakthrough in a deal for Russian Federation to participate in a production cut by the OPEC oil cartel next month.