The general coordinator of the left-wing United Left, or IU, coalition, Gaspar Llamazares, took part in the protest and told reporters that demonstrators wanted to "reject NATO and its policy of war, which is the only policy it has".
Llamazares said "new installations for war" were being built in Spain in violation of the terms under which it became a member of the alliance.
Demonstrators called for the withdrawal of Spanish troops from Lebanon and Afghanistan,where they are participating in NATO-led operations.
The spokesman for Seville's Social Forum, Jesus Lara, said another protest march was planned for Thursday, the first day of the gathering of NATO defense ministers.