On the Earth Science Reference Table if you look at Inferred Properties of Earth's Interior on page 10, you will see that the temperature of the Asthenosphere or Plastic Mantle as it is also known ranges from 1000 degrees Celsius to 2500 degrees Celsius--see link below. The Asthenosphere lies below the Lithosphere and it has a very high temperature. It is believed that the rigid Lithosphere floats above the slowly moving Asthenosphere causing thetectonic plates to move on Earth.
While doing my research it seems like there are many different answers. One source says: 1,400 degree Celsius to 3,000 degree Celsius, while another says 300-500 degrees Celsius.