They made their vow on a media conference call on Wednesday, as the US$2.3-billion Oklahoma-to-Texas stretch of the line officially began shipping crude oil.
Jane Kleeb of Bold Nebraska, a group fighting the pipeline, says the startup marks a sad day as it poses risks for those who live along the route.
The northern portion of the pipeline has been held up by concerns over potential risks to drinking water supplies and climate change.
TransCanada and the Canadian government are awaiting a decision from the U.S. State Department and President Barack Obama, whose approval is required because the northern portion crosses the border.
In the meantime, TransCanada has pushed ahead with construction of Keystone XL’s southern portion, which connects a major oil hub in Oklahoma with refineries in Texas.