Face coverings are now mandatory in shops and in enclosed public places in Northern Ireland.
The ruling was brought forward 10 days after it was revealed the R number could be as high 1.8.
The rise has resulted in so called wet pubs being told they will not be able to reopen until at least September. Retailers have urged the public to respect new rules that face coverings must be worn in all public enclosed spaces.
People can face fines for not wearing a face covering in shops and other enclosed settings unless the individual is unable to.
The legislation does not apply to children under 13, or those who cannot wear a mask because of health or disability reasons.
Breaches of the new rule can result in a fixed penalty notice of £60 which will be reduced to £30 if paid within 14 days.
We are confident most people will comply, but those who do not wear a face covering without a reasonable excuse risk being issued with a fixed penalty notice by the police.
Announcing the measure agreed by the Northern Ireland Executive last Thursday, First Minister Arlene Foster said it was about trying to give confidence to people who feel vulnerable as the coronavirus pandemic continues.
In a joint statement on Sunday evening, the Executive described the use of face coverings as a "vital defence in our battle against Covid-19".
The statement added: "This is about looking after each other and doing everything we can to stay on top of this virus. It will help to save lives.
"Now that the lockdown has been eased, we must keep following public health advice, whether that be wearing face coverings, maintaining social distancing or washing our hands."
Meanwhile, retailers say they will support the new regulations, but that responsibility lies with customers.
This is not just about a legal responsibility - it's about protecting each other. Please wear one - and encourage your family, friends and colleagues to do likewise.
Aodhan Connolly, director of the Northern Ireland Retail Consortium, said: "Retailers are doing all they can to support necessary safety regulations and will play their role in communicating and encouraging the Executive's new policy on face coverings.
"While enforcement of this policy will be handled by the police, the ultimate responsibility remains with customers who must ensure that they wear a face covering when going into stores.
"Our shopping experience is changing, and we ask customers to be respectful and considerate when the new rules come into force today.
"We would urge everyone who can wear face coverings to abide by the law and do so. Everyone has a role to play in ensuring compliance."
Mr Connolly added: "We look forward to further Executive communications on this to ensure everyone understands the reasons for these changes. It is vital that consumers are confident enough to return to their high streets, as demand remains low despite the lifting of lockdown.
"If sales and footfall fail to recover quickly, Government should urgently consider steps to boost demand or else we will see many more shop closures and subsequent job losses."