A Massachusetts coalition of small businesses, elected officials, and labor unions has called on the state to force online giant Amazon.com Inc. to collect sales tax on purchases.

The Massachusetts Main Street Fairness Coalition wrote a letter Thursday to Amy Pitter, the state’s Department of Revenue commissioner, demanding action in correcting what they say is a ‘‘disparity in state sales tax collection.’’

In the letter, the group urges the commissioner to force the company to collect the state’s 6.25 percent sales tax by July 1, saying local businesses are struggling to compete with tax-free online merchants.