RIYADH (AFP) - "The whole world is watching," a voice off-screen told Saudi Arabia's King Salman as he opened the annual G-20 summit on Saturday (Nov 21), in what might be 2020's highest-stakes video call.

The virtual "gathering" of the Group of 20 (G-20) rich nations seeks to tackle the coronavirus pandemic and the faltering global economy - but in a scaled-down format and without the behind-the-scenes interactions that are the lifeblood of global diplomacy.

Here are five things to know about the summit:

1. Face time

The virtual summit - Plan B when the coronavirus made a physical meeting impossible - deprived Saudi Arabia of the chance to invite the world to the grand coming-out party it had hoped for after a whirlwind period of change.

De-facto ruler Crown Prince Mohammed bin Salman, who became heir to the throne in 2017, has upended the ultra-conservative kingdom - revamping the economy, allowing mixed-gender events, and extending greater freedoms for women, including the right to drive.

But the lead-up to the summit brought negative headlines, including on the detention of opponents in an ongoing crackdown.

"The G-20 this year will be a disappointing one overall for Saudi Arabia, as a virtual conference will not showcase the kingdom's progress in the ways Riyadh hoped," said Mr Ryan Bohl, of US geopolitical think tank Stratfor.

2. The Trump factor

United States President Donald Trump went golfing after making a brief appearance at the cyber summit.

A source with access to the virtual sessions, which were closed to media, reported that Mr Trump had "said that he had done an absolutely incredible job during his term, economically and with the pandemic".

After his moment in the virtual limelight, Mr Trump was substituted by Treasury Secretary Steven Mnuchin as the other world leaders had their say.

The outgoing US President's motorcade was greeted at the entrance to one of his golf complexes by a protester who waved a sign adorned with a slice of bread and the words, "Face it. You're Toast".

3. Tech headaches

After a year of Zoom calls for work and play, the world is familiar with the perils of video-conferencing technology used to circumvent travel restrictions and social distancing requirements.

King Salman's opening comments appeared to go live earlier than planned, and the feed quickly cut away after an aide's whispered warnings to the monarch.

Other world leaders appeared in thumbnails on the online platform, clustered around King Salman and MBS, with some apparently struggling with the virtual format.

An aide to Chinese President Xi Jinping brandished a remote control at a screen.

Mr Angel Gurria, Secretary-General of the Organisation for Economic Cooperation and Development (OECD), quickly put away his phone after he realised he was on screen.

Germany's Angela Merkel, who marks 15 years in power on Sunday, appeared in her trademark pose - serious and focused.

Apart from the glitches, Mr John Kirton, director of the Canada-based G-20 Research Group, said the event would be poorer without spontaneous interactions over "summit tables, coffee breaks, corridors or hotel gyms" typical of past meetings.

4. Going green

Private jets, a common sight at past summits, were absent this year - slashing the event's carbon footprint, along with the lack of leaders' motorcades which would ordinarily grind the host city to a halt.

However, one green innovation, a new metro line slated to open in Riyadh for the summit, did not come to fruition in time as planned.

5. Say cheese (and dates)

The lack of a traditional leaders' "family photo" brought home the reality of coronavirus restrictions on movement and gatherings.

The encounters have defined several past gatherings, including the famous high-five between Russian President Vladimir Putin and Crown Prince Mohammed at the G-20 in Buenos Aires in 2018.

Instead, a composite shot was projected onto ruins in the historic town of Diriyah outside Riyadh on the eve of the summit.

In the shadow of the giant projection, media personnel were treated to a glitzy spectacle of traditional musicians as they tucked into date sorbet.