Twenty people were arrested on Friday night and in the early hours of Saturday morning in Letterkenny.
Those arrested will face charges of drink driving, public order offences and motoring offences.
Thousands of people have come from all over for the Donegal International Rally which began on Friday afternoon and concludes on Sunday evening.
The Port Road and Main Street were blocked blocked off last night - only taxi drivers were allowed through the town's main thoroughfare. The same will be the case tonight.
The town of Letterkenny was extremely busy last night.
A large garda presence was in place along the Port Road and Main Street areas.
Speaking to the Donegal Democrat, Garda Inspector Michael Harrison said while it was a busy night in the Cathedral Town, the vast majority of those who had come for the rally were extremely well behaved.
"While we had 20 arrests for various offences, the large numbers were extremely mannerly and courteous towards the gardai. Tonight is going to be another busy night and we are expecting something similar in terms of arrests," he said.
"Again, we woulds appeal to everyone to obey the rules of the road, leave the rallying to the experts in the rally and co-operate with the gardai who are there to ensure that everyone has a safe and enjoyable weekend in Letterkenny," he added.