(Adds Amazon statement in seventh paragraph)

By Brad Haynes

SAO PAULO, March 28 (Reuters) - Amazon.com Inc dove into a spirited debate about graffiti in Brazil's biggest city, drawing a pointed response on Tuesday from Sao Paulo's mayor, who called the company "opportunist" and challenged it to make a public donation.

Amazon kicked off the controversy with an online ad pegged to Mayor Joao Doria's "Pretty City" campaign, which has attracted both strong criticism and praise for painting over the city's street art and spray-painted tags.

"They covered the city with gray?" reads the minute-long commercial, which shows quotes from famous works of literature projected along major avenues painted over by the mayor. "We covered the gray with stories."

The campaign added fuel to the debate over whether Doria was defending public property or silencing artistic expression in a metropolis defined by the flamboyant graffiti that punctuates an otherwise monotonous urban landscape.

It also pitted Amazon against Doria, a wealthy publisher and former reality TV star whose stunning first-round victory in last year's mayoral election has fed rumors that he could enter the wide-open 2018 presidential race.

"I watched Amazon's commercial using the image of Sao Paulo to sell its products," Doria wrote in a response on Facebook, challenging the online retailer to donate books to city schools. "There are many ways for Amazon to act as a true citizen rather than an opportunist."

Amazon replied with a Facebook post promising to donate hundreds of its Kindle e-readers to "institutions promoting culture and education," without giving further details.

Amazon media representatives in Brazil declined to comment.

The company has been slower than other tech giants to embrace the Brazilian market, where it arrived in 2012 with the Kindle, which faces local competition from Livraria Cultura's Kobo and Livraria Saraiva's Lev.

Amazon began selling physical books in Brazil in 2014 and brought its streaming video service to the country in December.

(Reporting by Brad Haynes; Editing by Peter Cooney and Bill Trott)