The blocked sites included bet365, Ladbrokes and 888.Com as reported today. The law came into effect yesterday.
The objectives of regulating remote gambling are to maintain law and order and to minimise the potential harm of remote gambling, especially to young persons and other vulnerable persons, said the Ministry.
The Ministry stressed that any attempt to make payments to these betting firms will be blocked.
The MHA said it will regularly review the sites' list, which has been sent to Internet service providers here.
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The blocked sites contained prohibited material, said a message posted on the sites.
Data would be sought from research firms to track the websites' visitor statistics, according to the media reports.
Singapore authorities would also work with gambling regulators in other jurisdictions, such as France, to share and learn from "best practices", said the MHA.
Unlicensed online and phone gambling is an offence under the Remote Gambling Act which was passed last October.
Gamblers face up to six months in jail or a SGD5,000 fine and those guilty of luring people under 21 face stiffer penalties.
Local media reports said several betting firms' mobile websites, which are optimised for use on phones and tablets, were still accessible yesterday.