The nine-day camp, at the franchise's home venue Wankhede Stadium, is to last till April 2.
West Indian Nicholas Pooran is the first international player to join the camp.
Mitchell Johnson, Jos Buttler, Mitchell McClenaghan, Lasith Malinga, Tim Southee, Kieron Pollard and Lendl Simmons, from the rest, would join the camp in the coming days.
"Other foreign players will join the camp as and when they are free of their current engagements," MI sources said.
Pacer Jasprit Bumrah and all-rounder Hardik Pandya went through the paces along with the latest MI picks in the IPL auction - Karn Sharma, Krishnappa Gowtham and Saurabh Tiwary - along with the retained players Jagadeesha Suchith, Vinay Kumar, Nitesh Rana, Deepak Punia and Jitesh Sharma.
Existing team of support staff, including Robin Singh (batting coach), Jonty Rhodes (fielding coach) and Shane Bond (bowling coach), were present on the opening day along with Jayawardene.
Mumbai Indians, champions in 2013 and 2015, open their campaign against Rising Pune Supergiant on April 6 in Pune.
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