Activists suing the city of Minneapolis over a deal to build a park on the east edge of downtown received a major blow Friday afternoon when a Hennepin County judge ordered them to post a $10 million surety bond.

Judge Mel Dickstein said plaintiffs Paul Ostrow, Stephanie Woodruff and Dan Cohen must post the bond before Thursday. Ostrow acknowledged in an interview that this won’t be possible, and said they are exploring appeal options.

“We think there are some clear factual errors and procedural errors in the court’s decision,” said Ostrow, an Anoka County prosecutor and former Minneapolis City Council president. “But whether there are issues that we can appeal, and how we might do that, is something that we have to determine.”

The group had argued that only the city’s Park and Recreation Board has the authority to own and operate a park. At issue is a 1.66-block park adjacent to the new $1 billion Vikings stadium, which is the centerpiece of a $400 million development project proposed by Ryan Cos.

That project includes two office towers for Wells Fargo & Co., up to 400 apartments, retail shops and restaurants, a parking ramp and the park on land now owned by the Star Trib­une. The land deal with the newspaper is supposed to close by Jan. 24.

Attorneys for the city had argued in court that the uncertainty surrounding the litigation could imperil the city from issuing up to $65 million in bonds to pay for the parking ramp and the park. They requested the surety bond to cover any losses if the suit fails.

“Under the facts of this case, a surety bond should be required,” Dickstein wrote. He said the lawsuit “challenges the city’s authority to contractually bind the city to design, construct and maintain a park as part of its commitment to the Downtown East developer.”

In addition, Dickstein said “the city may suffer a loss from the mere pendency of this action,” and that the “plaintiffs are not likely to succeed on the merits” of the case.