Estoy usando ubuntu 12.04 y estoy tratando de instalar virtualenv, pero de repente recibí este error.

samuel@sampc:~$ pip install virtualenv
Downloading/unpacking virtualenv
  Running setup.py egg_info for package virtualenv

    warning: no previously-included files matching '*' found under directory 'docs/_templates'
    warning: no previously-included files matching '*' found under directory 'docs/_build'
Installing collected packages: virtualenv
  Running setup.py install for virtualenv
    error: could not create '/usr/local/lib/python2.7/dist-packages/virtualenv_support': Permission denied
    Complete output from command /usr/bin/python -c "import setuptools;__file__='/home/samuel/build/virtualenv/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --single-version-externally-managed --record /tmp/pip-Z2v_fR-record/install-record.txt:
    running install

running build

running build_py

running install_lib

creating /usr/local/lib/python2.7/dist-packages/virtualenv_support

error: could not create '/usr/local/lib/python2.7/dist-packages/virtualenv_support': Permission denied

----------------------------------------
Command /usr/bin/python -c "import setuptools;__file__='/home/samuel/build/virtualenv/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --single-version-externally-managed --record /tmp/pip-Z2v_fR-record/install-record.txt failed with error code 1
Storing complete log in /home/samuel/.pip/pip.log

¿Alguien tiene una idea sobre mi caso?