Members of Unite held demonstrations at the Mini plant in Cowley, Oxfordshire, and the Hams Hall engine plant in the West Midlands.
The union said closure of the BMW occupational pension scheme could see some workers lose up to £160,000 in retirement income.
A ballot for industrial action is being held, with the result due at the end of the month.
A BMW statement said: "BMW Group has always prided itself in providing excellent pensions for all its staff and wants to act now to protect future pension provision.
"Discussions are ongoing and a further meeting with our employee representatives will take place later this month."
Unite national officer Tony Murphy said: "Workers will be protesting and encouraging their colleagues to back industrial action and send a clear message to BMW bosses that they will not tolerate the firm's pension robbery.
"The UK workforce has contributed massively to BMW Group's record sales and revenues and deserves better than BMW's broken pension promises and pleas of poverty.
"We would urge BMW to recognise the vital role the world-class UK workforce has played in the company's success and work with Unite to find a solution which safeguards car workers' pensions and works well for the business."