A group of US volunteers that has been patrolling the frontier with Mexico to stop illegal immigrants has started building a fence along the border.
The Minutemen held a ceremony to mark the start of work
The Minutemen group plans to erect a combination of barbed wire, razor wire and steel barriers along a 10-mile (16km) stretch of privately-owned land.
Hundreds of volunteers gathered in Arizona for an inauguration ceremony.
The Minutemen are allowed to report illegal crossings to border police but have no right to arrest suspects.
Human rights groups have accused the Minutemen of xenophobia towards illegal immigrants - but the group denies this.
US President George W Bush backs plans to curb illegal immigration by extending a fence along the Mexican border and increasing the number of patrols along it.
The question of easing - or tightening - curbs on illegal immigrants is one of the most divisive issues in US politics, provoking fierce debate among Democrats and Republicans.
The Minutemen have long campaigned for a secure fence along the border.
The group has now begun building its fence at the site where it conducted its first patrols in November 2002.
"Many have talked of building a secure fence between Mexico and the United States," the group said on its website, adding that it is now "taking action" and "doing the job the federal government will not do".
A spokeswoman for the group told the Associated Press news agency it would take three weeks to build the fence, costing an estimated $100,000 (£53,900).
She said the group had already raised $380,000 (£204,700) to build more fences.
President Bush's plan for better border security is part of a package of measures approved recently by the US Senate.
But the package also contains provisions for a guest-worker programme and for offering citizenship to some illegal immigrants.
Some opponents say it is too soft and that all illegal immigration should be criminalised.
The Senate bill will have to be reconciled with a tougher immigration bill, backed by the House of Representatives, if any part of it is to become law.