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Charlie Neibergall, Associated Press
Democratic presidential candidate, Sen. Bernie Sanders, I-Vt., poses for a photo with supporters following a town hall meeting, Thursday, Sept. 3, 2015, in Grinnell, Iowa.

OTTUMWA, Iowa — Democratic presidential candidate Bernie Sanders said Thursday that his Iowa organization has picked up steam in recent months.

The Vermont senator and 2016 hopeful appeared at a campaign office opening in Ottumwa. People packed inside the building and spilled out onto the street, enthusiastically cheering his appearance.

"This is phenomenal," Sanders said, noting that the campaign has made significant progress in recent months. "These are all supporters, these are all people who are going to be volunteering and the challenge now is to make sure we can effectively put them to work."

Sanders' Iowa director, Pete D'Alessandro, said Sanders now has 15 offices and 54 paid staff in Iowa, with more hires expected soon.

Democratic front runner Hillary Rodham Clinton, who has been campaigning regularly in the state, has also built a sizeable Iowa operation that will continue to grow. She has 11 offices and 47 organizing staff, as well as additional professional staffers in the state.

A recent Des Moines Register/Bloomberg Iowa poll showed that Sanders is gaining on Clinton in the leadoff caucus state.

Sanders drew hundreds earlier in the day in Grinnell, where he spoke in a park about issues such as income inequality, campaign finance reform and college affordability. Crowds roared applause as he spoke from a gazebo in the center of the town that is home to Grinnell College, a small private liberal arts college.

Sanders thanked his supporters, saying: "this is a people's grassroots campaign. And that, my friends, is why we're going to win the election."

On Friday, Sanders plans to hold events in Muscatine, Tama and Cedar Rapids.

Clinton plans to return to Iowa Sunday.