The U.S Consulate is urging American citizens to avoid a protest march against the TPP tomorrow in Auckland.

It's issued a security message, saying even demonstrations intended to be peaceful can turn confrontational.

The alert is understood to be a relatively normal procedure, issued for large scale events in any country.

Three similar warnings were issued last year for rallies for Palestine.

Police think some 8000 people could attend the march from Aotea Square, down Queen Street and onto Commerce Street tomorrow afternoon.

Police Inspector Regan James said organisers have assured police it is being promoted as a family event. 

He said as with any event in the CBD, there may be some congestion so it might pay to avoid the area over the lunch period.

Police will be on hand to observe and ensure safety.