The protesters, mostly Communist Party members and supporters, rallied to denounce the Kremlin's plans to allow the Western alliance to use an airport in the Volga River city to help supply its troops fighting in Afghanistan.
Russian Foreign Minister Sergey Lavrov has said the transport hub would be for non-lethal cargo only. Such a route would allow NATO an alternative to transiting through Pakistan en route to its Afghanistan mission.
The Kremlin is supporting the idea to help reduce the threat of Islamic extremism and illicit drugs emanating from Afghanistan.
But the plans, which haven't been finalized, have drawn increasingly strident opposition from the Communists, who say they are incensed that a "base" run by the former Soviet Union's Cold War adversary would be allowed in the birthplace of Vladimir Lenin.
Last month, they demanded the issue be put to a referendum. The party's Web site said the Communists were demanding a vote in the Ulyanovsk Region on the question, "Do you approve of the deployment of a NATO base in the city of Ulyanovsk?" before the final decision is made.
They staged a rally Saturday in which 800 people demanded a halt to the plan as a danger to Russia's security, RIA Novosti reported.
The news service said the rally was led by Communist Party leader Gennady Zyuganov and included the participation of A Just Russia Party, the Left Front Movement and The Other Russia independent news Web site.
Zyuganov told protesters allowing NATO onto the Ulyanovsk site would be "a betrayal of Russia's national interests," The New York Times reported.
Russian authorities, however, have dismissed those concerns, saying the NATO shipments would be subject to customs checks.
"I assure you that nothing unusual, nothing that doesn't correspond to our national interests, is happening here," Russian President-elect Vladimir Putin has said, while Lavrov added, "We are not talking about a military base."
Deputy Prime Minister and former NATO envoy Dmitry Rogozin has called the concerns "nonsense" and a "phantasmagoria."
But such assurances haven't mollified the protesters.
The Communists have asserted in a letter to Rogozin that the transit hub is more dangerous for stability in Russia than the threat of Ukraine's Orange Revolution, RT Television reported.
"The (NATO) alliance skillfully disguises its military interventions as humanitarian and its military bases as purely civil facilities," the letter said. "The patriotic rhetoric that we used to hear from you in Brussels, has suddenly changed to statements that are in line with NATO policies."
NATO, they warned, has always "extorted unilateral concessions" from Russia.
Saturday's protest also resulted in the arrest of Left Front leader Sergei Udaltsov. He told RIA Novosti he was detained by local police after the rally.
"When the rally ended, I and my friends were going to a hotel to hold a press conference about our future plans. But police officers came up and told me ... a woman filed a complaint that I inflicted bodily injuries to her," Udaltsov said.
An interior affairs official told the news agency Udaltsov had been "invited to the police office to give explanations concerning the application from the woman on the fact of injury."