A major conference that brought Mayor Martin J. Walsh national attention was funded by companies and groups that do business in Boston, and watchdogs are blasting the sponsorships as influence-peddling efforts aimed at “greasing the skids.”

Walsh hosted the four-day annual meeting of the U.S. Conference of Mayors this month, which brought more than 200 mayors from around the nation to Boston for a series of meetings during the day and parties at night. A review of past conferences’ finances suggests the sponsorships could total in the millions.

The conference saw Walsh lead a high-profile march in the city’s Pride Parade and hold a fireside chat with Facebook COO Sheryl Sandberg, where she announced a major jobs-training initiative.

Walsh spent $12,000 of city money on promotional materials such as pens and notebooks, but the rest of the conference — which included a night out at the Museum of Fine Arts and a party at District Hall in the Seaport — was paid for by sponsorships. Nearly 100 local and national corporations, as well as unions and museums, sponsored or gave in-kind services for the event.

A Walsh spokeswoman said the mayor does not know how much any of the sponsors donated. A spokeswoman for the conference did not return repeated requests for information about how big a donation was required for each level of sponsorship — Presenting, Gold, Silver, Bronze and Sponsor — or the overall cost of the convention. But tax returns of the committee that organized the group’s annual meeting in Dallas in 2014 suggest the shindig is a pricey affair, listing $4.4 million in sponsorships received.

In Boston, megadeveloper Suffolk Construction and the Metropolitan Building Trades Council — which Walsh once headed — were both “Presenting” sponsors, the highest level. Seven other unions were also sponsors, and Airbnb, which was opposing Walsh’s short-term rental proposal, was a “Title Sponsor.”

Organizations that have business with the city sponsoring events is “completely inappropriate,” said David Tuerck, executive director of the Beacon Hill Institute.

“This should be funded by the mayors attending it, not by contractors who want to be noticed by the mayor for their generosity,” Tuerck said. “It has the effect of pushing the agenda of sponsoring entities ahead of their competitors, greasing the skids for these supporters when they come to City Hall.”

“Our bottom line is transparency— every donation needs to be made in the open in a timely manner with the donor and the amount,” said Pam Wilmot, executive director of good-government group Common Cause.

A Walsh spokeswoman insisted the mayor won’t be swayed by sponsorships.

“No donations, whether political, charitable, or sponsorships, influence the mayor’s decision making on official city business,” City Hall spokeswoman Samantha Ormsby said. “USCM served as a special and unique opportunity to share best practices with fellow mayors and for Boston to showcase its progress, and we have a lot to be proud of.”

A review of sponsors for the 2017 annual meeting in Miami and 2016 annual meeting in Indianapolis shows that local businesses were eager to support the events. Miami saw 60 sponsors, including the cruise ship and hospitality industries, while Indianapolis had 68 sponsors including area tech firms and attorneys.

An Airbnb spokeswoman told the Herald the company has sponsored USCM meetings since 2016. Suffolk and the Building Trades Council did not return immediate requests for comment.