Had alternator replace under warranty but when driving it accelerated rapidly and I was barely able to stop it.

I had picked up the car from being serviced - a faulty alternator was replaced under warranty. A few hours later I was driving the highway and less than 10 minutes later the car suddenly accelerated to between 6-7,000 RPM's. I quickly took my foot off the gas pedal and it just kept going. It wouldn't stop. I tried pumping the brakes and it wasn't until the revs dropped to just under 5,000 that I was able to slow it down, take a chance and put it in neutral and just as it was slowing enough for me to pull off onto the side of the road, the engine started roaring again and I pumped the brakes and slid it into park. It was a miracle that I wasn't killed or that this hadn't happened just as I was driving through town where there was lots of traffic. It has since been towed and I am waiting for a verdict from the service station. Has anyone else ever experienced this situation? It was the most scared I have ever been after 50 years of driving.