Criminals could be trying to sabotage this weekend's Macau Grand Prix, officials said last night.
The warning came after two youths on a motorcycle threw a bag containing nails and a firecracker on to the circuit and a grenade was found nearby.
Cars practising for the Guia Race ran over the bag, scattering nails across Fisherman's Bend at about 3.15pm. Unconfirmed reports said at least two cars suffered tyre damage.
The circuit was closed and police and ammunition experts examined the scene, discovering a deactivated Russian-made grenade on the street next to the track. A Macau Security spokesman said: 'We have no evidence of any explosives found with the package thrown on to the track and none to say that the grenade is connected to the parcel of nails.
'It looks like someone is trying to destabilise the event but they won't succeed.
'I have just spoken to the organisers and this incident did not interrupt the normal running schedule.' The spokesman said security would be tightened.