Greek police have clashed with protesters in the capital Athens as unions stage a general strike against government austerity measures.

Protesters threw petrol bombs and police responded with tear gas as the violence flared outside parliament.

The day of action has grounded flights, disrupted public transport and closed schools across the country.

It is the seventh general strike this year following tough reforms needed to receive a 110bn euro (£84bn) bail-out.

The BBC's Malcolm Brabant explains one theory as to what sparked the violence.