BRATISLAVA, Slovakia (AP) — Thousands of Slovaks rallied Tuesday to protest a military defense treaty between their nation and the United States, which are both members of NATO.

Waving national flags and banners such as “Stop USA Army," the protesters gathered in Bratislava in front of Parliament, where lawmakers were debating the Defense Cooperation Agreement. Police prevented some protesters from entering the building.

The parliamentary debate was obstructed by opposition lawmakers, who used whistles to prevent others from speaking. They also seized an Ukrainian flag unveiled by the coalition lawmakers, after pouring water on it and a lawmaker who was holding it.

Ukraine's embassy in Bratislava protested the incident, which took place amid fears of Russia’s possible invasion of Ukraine as more than 100,000 Russian troops massed in areas near Ukraine's border.

A vote by Slovak lawmakers on the measure is expected on Wednesday.

The protest united the Slovak opposition, including the far-right People’s Party Our Slovakia and the leftist Smer-Social Democracy of former populist Prime Minister Robert Fico. Fico supported the treaty when he started the negotiations on it with the U.S. in 2018 but has since turned into a vocal opponent.

The agreement has been supported by Prime Minister Eduard Heger's four-party ruling coalition government, which said it will “significantly enhance our security.”

The opposition claims it would compromise the country’s sovereignty, make possible a permanent presence of U.S. troops on Slovak territory, enable a deployment of nuclear weapons in Slovakia and provoke Russia. The Slovak and U.S. governments have rejected those charges.

The treaty allows the U.S. military to use two Slovak air force bases — Malacky-Kuchyna and Sliac — for 10 years while Slovakia will receive $100 million from the U.S. to modernize them.

Any particular future deployment of U.S. forces will still need approval by the Slovak government and Parliament.