Police are negotiating with a man on the gantry

Mother-of-one Jayne Woodman, 36, a human resources manager from Swansea, climbed the gantry of the bridge with Jolly Stanesby, a registered childminder from Ivybridge in Devon.

A Devon and Cornwall police spokesman said she came down after some hours and was arrested.

Mr Stanesby is still on the bridge.

"She feels very strongly about this Graeme Cook, F4J Wales Co-ordinator

The protesters are both members of the pressure group Fathers 4 Justice, which is campaigning for equal parenting rights and a legal presumption of contact for parents and grandparents following separation or divorce.

Long traffic tailbacks resulted on both the Devon and Cornwall approaches to the bridge when the pair blocked the carriageway with two cars around 0700 GMT and climbed on to a gantry.

The bridge was fully reopened at 0930 GMT.

Ms Woodman is the first woman to participate in such a protest, the latest in a series of high-profile stunts designed to highlight the plight of fathers and their families separated from their children.

Mr Stanesby, 35, is a veteran of similar previous protests, and was one half of the "Batman and Robin" duo who staged a protest on the roof of the Royal Courts of Justice in October.

He said he had enough supplies to stay up there for a week.

A similar demo by the group took place on the bridge in October last year.

Ms Woodman is the partner of F4J Wales Co-ordinator Graeme Cook, who said: "She feels very strongly about this.

"As the single mother of a young child herself, she cannot understand how other mothers would want to restrict their children's relationship with their father or even cut him out of their lives altogether, which is what is happening on a catastrophic scale in the UK with the collusion of the family courts."