A few policemen's hats were knocked to the ground by protesters as they jostled and shoved at the Princes St entrance.
Protesters linked arms in an attempt to keep National Party supporters out of the restaurant.
Some supporters pushed their way in. Some repaired to the Japanese restaurant next door to watch the melee, or walked to the Moray Pl entrance.
Many were annoyed, one telling the ODT he respected the right to protest, but "I also have a right to go and have dinner".
The man, who declined to be named, said he had concerns about the TPP, and hoped Mr Key would talk about the controversial deal during the dinner.
Police closed part of Princes St and called for reinforcements.
By 7.30pm, the protesters had dispersed.
Earlier, a smaller group was at the art gallery, again hoping to see Mr Key, who did not appear in public.
Protesters congratulated Class Act recipients on their achievement, and the pupils and parents did not seem perturbed as they filed out of the building.
Prof Richard Jackson told the ODT he was protesting because of the "deeply undemocratic" nature of the TPP.
Earlier yesterday, at a press conference at Dunedin Airport, Mr Key said the Government was "never going to sign a deal unless we think it's in New Zealand's best interest".
"There won't be a collapse of Pharmac or alterations to the way Pharmac operates.
"There's a lot of misinformation that when you get a chance to talk to people about it, often it reassures them, but it's not going to reassure everybody."
Senior Sergeant Brian Benn said no-one was arrested at the protest.
Police called in units from around the city including a dog handler. However, the dog was not required.
"There was a noisy group there but there were no issues and the Prime Minister's plans were not disrupted," he said.
Earlier yesterday, Mr Key cut the ribbon at the official opening of George Street Normal School's $1.2 million new modern learning environment.
He congratulated the Dunedin school for its fundraising effort. Only a "small amount" of funding - about $150,000 - was contributed by the Government.
Mr Key said he had opened several similar facilities around the country, and the multipurpose libraries were a great way for children to learn.
"It will get a massive amount of use over the years ahead, so well done," he said.
- Additional reporting Tim Brown
