Petitions about the NHS, trophy hunting and access to life-saving drugs were among the most popular on the Change.org website during 2019, figures have shown.
The best-backed cause in petitions started and signed on the site in the UK during the last year saw more than 1.25 million people support calls to keep the health service out of any transatlantic trade talks between Prime Minister Boris Johnson and US President Donald Trump.
The petition was launched in June by NHS doctor Sonia Adesara following Mr Trump's visit to the UK.
In the second most popular campaign, nearly 600,000 people added their signatures to a call for a ban on lion trophy hunting imports into the UK.
Public opinion is thought to have been partly behind the announcement of the Animal Welfare (Sentencing) Bill in the Queen's Speech in October which said it will look to include "banning imports from trophy hunting".
The third most popular petition on the Change.org website saw more than 400,000 signatories call on the Government to make sure the UK does not leave the EU without some kind of Brexit deal.
The petition was started by Hackney councillor and charity pioneer Mete Coban, who was made an MBE in the 2020 New Year Honours list for services to young people.
Elsewhere, hundreds of thousands of people signed petitions calling for pet theft to be made a specific crime in UK law, while others successfully backed a campaign calling for a drug to treat children with a rare genetic condition to be made available on the NHS.
Kajal Odedra, UK executive director of Change.org, said: "This year, millions of people have protected animals, tackled climate change and fought to protect social care and the NHS.
"The good news is that almost all of the most popular petitions have either won or made huge progress.
"It's a story of hope - when people come together, change is possible."
