The ''lockdown-like'' restrictions, which exclude essential services, became operational at 8 pm and will remain in force till 7 am on May 1.
Thackeray, in his address via social media to the people of the state on Tuesday night, had announced the sweeping restrictions, curtailing the movement of people and non-essential activities, to prevent the spread of COVID-19 in the country's worst-hit state.
Section 144 of CrPC, prohibiting assembly of five or more people at one spot, will be in force during the period, Thackeray had said, but refrained from terming the new curbs as a lockdown.
In a video conference meeting on Wednesday with his cabinet colleagues, divisional commissioners, district collectors, municipal commissioners and police officials, Thackeray stressed on effective implementation of the measures.
There should not be any confusion about the orders issued by the state. If anyone has doubts, the official should contact his or her superiors or the department concerned.
''We can not afford to be callous and let the coronavirus infection spread further, the chief minister said.
Thackeray said efforts are on to make available medical oxygen which is needed in treatment of critical COVID-19 patients.
We have seen a mutation in the coronavirus strain.
It is spreading at a higher speed and affecting the youth in large numbers.
''Efforts are underway on a war footing to make oxygen available for patients. Doctors should get themselves updated about the treatment of COVID-19 patients, he said.
He asked jumbo COVID-19 care centres and hospitals to undertake safety measures, including fire audits of their buildings.
All jumbo centres as well as hospitals should complete their fire audits and building strength assessment to avoid damage from pre-monsoon gusty winds,'' Thackeray said.
''There should not be any carelessness in implementing these instructions, said the chief minister.
All essential services, including medical stores, hospitals, supply of medicines, among others will remain operational, according to a government notification.
The local transport, including Mumbai's suburban trains, will function only for personnel involved in essential services, it said.
Activities such as the ongoing vaccination drive, oxygen production, home delivery of food, operation of e- commerce firms, work at construction sites with residential facility and industrial units, where accommodation or transportation of personnel is available, will also remain unaffected, the notification said.
All banking and financial services, the RBI and its affiliated offices, insurance offices and pre-monsoon works will operate as usual.
However, there will be a complete ban on religious, social, cultural and political functions in the state till May 1, the notification said.
The number of participants at marriage and funeral has been capped at 25 and 20, respectively.
Barber shops, saloons, spas, schools, colleges, coaching classes, beaches, clubs, swimming pools, gyms, drama theatres and cinema halls will remain shut during the next 15 days, according to the notification.
Though roadside eateries are allowed to operate during the period, buyers can not consume food on the street and only takeaway will be available, it said.