BANJA LUKA, Bosnia-Herzegovina (CNN) -- Busloads of Bosnian Serbs loyal to indicted war crimes suspect Radovan Karadzic were blocked from entering Banja Luka Monday after NATO became convinced they were part of a coup attempt to oust Bosnian President Biljana Plavsic from power, senior NATO officials told CNN.

NATO got wind of a plot as thousands of supporters of Karadzic headed from his Pale region stronghold toward Banja Luka, where Western-backed Plavsic is based.

Plavsic and Karadzic have been locked in a power struggle for months.

British, American and Norwegian NATO troops found small stashes of weapons concealed among the 70-odd buses that were headed into the Plavsic-supported city.

A senior NATO source said that a small group of infiltrators with uniforms, weapons and false identity papers were ready to take over the local Banja Luka police station. They had planned a coordinated strike that would have replaced the democratically-elected Plavsic with Karadzic supporters.

The busloads of supporters had said they were going to Banja Luka to hold an election rally.

The supporters had several scuffles with NATO troops along the way as the troops stopped the vehicles for inspections.

The real intention of the rally was to create a diversion as thousands of rock- and stick-wielding demonstrators engaged police. A smaller, armed group would have staged a coup and toppled the democratically elected government, the source said.

Hard-line Serbs threatened, taunted and then overran about 25 Norwegian soldiers who had stopped them for an inspection. The hard-liners then poured gas over the Norwegian's vehicles and unsuccessfully tried to set them on fire. No one was hurt in the incident.

American and British troops, who had considerably more firepower, refused to let those and other buses enter the city as punishment for running past NATO checkpoints.

At one point, American troops detained about 30 buses just outside the city, and British troops detained another 54 buses in a tense standoff near the central Bosnian town of Prnjavor.

Only some 150 supporters of Karadzic's ruling SDS party were present at the rally, which had been banned by pro-Plavsic police.

The small rally did take several violent turns as the hard-liners smashed cars and tossed rocks at buildings. There were no reports of fatalities.

At almost midnight Monday NATO officials said they were hoping the Karadzic supporters would tire and go home. Some apparently had.

A NATO source said one protester claimed he had been paid 200 German marks to take part in the rally.

Military Affairs Correspondent Jamie McIntyre Reuters contributed to this report.