"Mithi is around the danger level because of the rains. Please cooperate with the BMC, NDRF, police and other authorities, when they come to help," Thackeray wrote on micro-blogging site Twitter.
"300 mm is a test for any city, and neither anyone of us, nor government agencies will shy away from responsibility. We will continue working on streets," Thackeray tweeted.
"There seems to be a heavy cloud build up for rains coming our way. Will request all not to be around the coastline and cooperate with police.
"1916 is the BMC disaster control helpline, please do spread this and use this to report and coordinate with BMC, Police, armed forces, fire brigade," he said in a series of tweets.
Thackeray said that as per statistics, Mumbai has received an average of 15 days rain in 24 hours, and that the weather systems have predicted that it won't stop raining till tomorrow evening.