Seven frac sand protesters were arrested Wednesday morning by Brooklyn Center police after they climbed onto the roof of a tour bus waiting to take members of an industry group on a field trip to sand mining and processing sites in western Wisconsin.

At sunrise outside the Earle Brown Heritage Center, the group of Catholic Worker activists leaned a ladder against the bus and carried banners, coffee and water to the roof. Fellow protesters whisked the ladder away and the occupation lasted until about 8:30 a.m., delaying by about an hour the field trip planned by the Conference on the Silica Sand Resources of Minnesota and Wisconsin.

After police summoned their own ladder, the protesters heeded demands to get down, said protester Daniel Wilson, 24, of Winona, Minn. It was the second protest in three days outside the conference, where geologists and mining professionals gathered to share technical information about the burgeoning frac sand industry in western Wisconsin and southeastern Minnesota.

"We wanted to put forth the message that our tragedy is not your tour," Wilson said after he and the others were booked and cited for unlawful assembly.

More than 75 mines and frac sand processing facilities have opened in western Wisconsin in the past two years in response to a national oil and gas drilling boom that uses sand in a drilling process known as "fracking." Minnesota, too, is dealing with a crush of applications for new frac sand facilities, and several counties on both sides of the border have imposed temporary moratoriums.