Pop and classical musicians from the United States, Asia and the Middle East, all performing from their homes, appeared on the One World: Together at Home live stream due to run for six hours to pay tribute to frontline workers battling the health crisis around the world.
Jamil, known for her TV series The Good Place, told viewers the event was meant to be "a moment of respite".
"Put your wallets away. This is not a fundraiser. We are just here to give you an incredible show with amazing live performances," she said.
The livestream's first hours featured the likes of British football star David Beckham, American R&B singer Jennifer Hudson, Hong Kong singer Eason Chan, India's Vishal Mishra and Emirati musician Hussain Al Jassmi. That will be followed by a two-hour concert broadcast on Saturday night across multiple television channels and featuring stars including The Rolling Stones, Paul McCartney, Taylor Swift and Billie Eilish.
The event, curated by Lady Gaga and organised by the World Health Organisation (WHO) and the nonprofit group Global Citizen, is the biggest celebrity effort so far to mark the coronavirus pandemic that has killed more than 150,000 people worldwide.
First to perform was US singer songwriter Andra Day, who sang the ballad Rise Up from her apartment.
One Direction star Niall Horan followed shortly afterwards singing with an acoustic guitar from his living room while US singer Adam Lambert gave a rendition of the Tears for Fears song Mad World. Contributions were also expected from Annie Lennox and Christine and the Queens.
Rita Ora urged viewers to stay safe and follow WHO recommendations, before singing I Will Never Let You Down.
The livestream opened with social media postings and news clips showing doctors and nurses being applauded by communities around the world, families greeting each other through windows, and acts of kindness in support of those who are isolated and frail.
Other segments highlighted the personal stories of doctors, nurses and grocery workers, poems submitted by children, and efforts by ordinary people to make masks and deliver food and medication to those in need.
"We are forever grateful for the sacrifices you have made to keep us healthy and safe," said Sex and the City actress Sarah Jessica Parker.
Celebrities also called on people to stay at home, wash their hands regularly, practise social distancing and put pressure on their political leaders to introduce widespread testing for the disease.
"We have to build stronger healthcare systems," said Jamil.
American actor Jack Black injected humour into the event with a fitness segment about flattening his own curve, featuring him attempting push ups, running, swimming and dancing.
While the special is not a public fundraiser, it is also aimed at encouraging philanthropists and companies to contribute to the WHO's Covid-19 Solidarity Response Fund. Corporate and other donors have already contributed some $US35 million ($58m), organisers said last week.