The Collaborative International Dictionary of English v.0.48:

Usurp \U*surp"\, v. t. [imp. & p. p. Usurped; p. pr. & vb. n.
   Usurping.] [L. usurpare, usurpatum, to make use of, enjoy,
   get possession of, usurp; the first part of usurpare is akin
   to usus use (see Use, n.): cf. F. usurper.]
   To seize, and hold in possession, by force, or without right;
   as, to usurp a throne; to usurp the prerogatives of the
   crown; to usurp power; to usurp the right of a patron is to
   oust or dispossess him.
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         Alack, thou dost usurp authority.        --Shak.
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         Another revolution, to get rid of this illegitimate and
         usurped government, would of course be perfectly
         justifiable.                             --Burke.
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   Note: Usurp is applied to seizure and use of office,
         functions, powers, rights, etc.; it is not applied to
         common dispossession of private property.
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   Syn: To arrogate; assume; appropriate.
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The Collaborative International Dictionary of English v.0.48:

Usurp \U*surp"\, v. i.
   To commit forcible seizure of place, power, functions, or the
   like, without right; to commit unjust encroachments; to be,
   or act as, a usurper.
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         The parish churches on which the Presbyterians and
         fanatics had usurped.                    --Evelyn.
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         And now the Spirits of the Mind
         Are busy with poor Peter Bell;
         Upon the rights of visual sense
         Usurping, with a prevalence
         More terrible than magic spell.          --Wordsworth.
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WordNet (r) 3.0 (2006):

usurp
    v 1: seize and take control without authority and possibly with
         force; take as one's right or possession; "He assumed to
         himself the right to fill all positions in the town"; "he
         usurped my rights"; "She seized control of the throne after
         her husband died" [syn: assume, usurp, seize, take
         over, arrogate]
    2: take the place of; "gloom had usurped mirth at the party
       after the news of the terrorist act broke"