To prevent workers from breaking the longest and costliest bout of industrial action the sector's history.
Some of the protesting strikers, clad in the green shirts of the Association of Mineworkers and Construction Union (AMCU), told Reuters they were there to block anyone from reaching the shafts.
The rival National Union of Mineworkers (NUM) said its members were unable to return to work because of AMCU intimidation. Four people have been murdered around the platinum mines in the last four days although police have made no arrests.
(READ MORE: S.African police deployed to platinum belt to protect miners)
"The miners cannot get to work because the intimidation is very high," Sydwell Dokolwana, NUM's regional secretary on the platinum belt, told Reuters.
London-listed Lonmin had been aiming for a "mass return" of workers in a bid to end a crippling 16-week strike that has also hit rivals Anglo American Platinum and Impala Platinum
A police spokesman told the eNCA television channel that officers were out in force and were on hand to escort those who wished to return to the mine gates.
"Buses and vehicles taking them to work will receive a police escort to make sure they are protected," police spokesman Thulani Ngubane said.
The companies have been taking their latest wage offer directly to AMCU's members after wage talks with the union collapsed three weeks ago.
The strike has halted 40 percent of normal global output and dented already sluggish growth Africa's most advanced economy.