In pictures: McCain's shock pick

Supporters at a Republican rally in Dayton, Ohio sang Happy Birthday to presidential candidate John McCain, as he prepared to announce his choice of running mate.

"She's exactly who I need, who this country needs, to help me fight to turn the same old Washington politics on its head", Mr McCain said of his choice, Alaska Governor Sarah Palin.

The 44-year-old mother of five, a relative unknown in nationwide US politics, introduced her husband, Todd, and other members of her family at the rally.

Thanking Mr McCain for choosing her as his running mate, Mrs Palin said she would be honoured to serve alongside him.

She also thanked Senator McCain "for refusing to hedge his support"' for US troops in Iraq, regardless of "political costs". Mrs Palin has a son, Track, who will shortly serve in Iraq.

Parts of Mrs Palin's speech were greeted by supporters gathered at the rally with cheers of "USA, USA".

She paid tribute to Hillary Clinton for the "determination and grace" she showed in her unsuccessful bid to become the Democratic presidential candidate.

Some analysts say Mrs Palin may have been chosen by the McCain campaign as a tactic to win over some Democratic women voters disappointed by Mrs Clinton's defeat by Barack Obama.