The fund-raiser was hosted by lawyer Charles Kopp of the prominent Philadelphia firm Cozen O'Connor.
A large contingent of police prevented the protesters from entering the building. No arrests or injuries were reported.
On Wednesday, nine Occupy Philly protesters were arrested during a sit-in at the Comcast Center. A 10th person was arrested outside.
At the Rittenhouse, the protesters posed for pictures with a large poster of Romney. Some carried mock campaign signs that read "Greed is Good! Romney-Gekko 2012," referring to the Michael Douglas character from the 1987 movie Wall Street.
Jamie Mondics, a spokeswoman for Keystone Progress, said the protest was organized Thursday.
"The fact that we were able to pull this off so quickly says a lot about how passionate people are here in Pennsylvania about tax cuts for the rich and corporate greed," Mondics said.
Patrick Miner, 26, said it was his first time joining an Occupy Philly protest. He said he had felt powerless during the debt-ceiling standoff between President Obama and Republicans in Congress.
"I think it provides a lot of people who feel powerless ... a chance to have a voice," said Miner, a structural engineer.
Pointing out that the fund-raiser cost $10,000 a plate, one protester declared: "You have to be a member of the 1 percent to get face time with a politician."
From the edge of the park, a couple from Chester County watched with curiosity, but the protest message wasn't resonating.
"I remember being 25 and out of work. You've got to pay your dues, work hard, take some risks," said Steven Hiscox, 52, president of a small business that trains people who want to become automotive technicians.
He and his wife, Tammy, were taking an after-lunch stroll through the square when they heard the ruckus and decided to see what was happening. Or at least he did.
"Frankly, I wanted to go in the opposite direction," his wife said.
The couple said the protesters were wrong to vilify businesses.
"Government won't solve all their problems," he said.
The protesters later marched around Center City and went back to Rittenhouse Square before returning to their base camp.
Contact staff writer Reity O'Brien at 215-854-2917 or firstname.lastname@example.org.
Inquirer staff writer Robert Moran contributed to this article.