Fears are mounting that packed mega-dinghies of migrants could start to land all along Britain's south coast after a boat completed a 70-mile Channel crossing to Portsmouth, aided by five French vessels and two helicopters.The inflatable carrying 140 asylum seekers set off from the Baie de Seine in Normandy yesterday, up to 300 miles by road south-west of Calais, as people smugglers launched a new route to the UK.Their arrival in Britain sparked chaos as protesters waded into the Solent to confront them, before around 500 then blocked roads near Eastney Beach in an attempt to stop buses taking the migrants away for processing.The flimsy 46ft dinghy had completed the 70-mile journey – the longest thought to have been attempted – across the treacherous and busy Channel to Portsmouth on Sunday, more than double the usual 34-mile crossing to Dover.Rather than intervene, the French deployed five rescue boats and two helicopters to shadow the vessel for ten hours until it reached UK waters, claiming it was too dangerous to intercept because of the numbers on board.British lifeboats, backed by a helicopter and fixed-wing aircraft, then took over for the final four hours of the crossing before bringing the migrants ashore on Sunday afternoon.Anti-migrant demonstrators were waiting in Portsmouth and dozens even waded into the sea in an attempt to turn them back.The group of protesters sprinted across Eastney Beach and into the water yesterday afternoon, chanting 'stop the boats' and 'enough is enough' as migrants arrived aboard a Coastguard vessel.The incident – the second in two days after hundreds of men wearing balaclavas blockaded the Port of Dover on Saturday – was the flashpoint which triggered around 500 demonstrators to block the road to try to stop the coaches leaving Portsmouth.After being dispersed by riot police in the early hours of this morning, some got into cars and followed the buses on their journey to the facility in Kent.Hampshire Police have said a small number of officers suffered injuries and have launched an investigation to identify the culprits. They added that a number of police, Coastguard and Border Force vehicles were damaged during the protest.It came as the Home Office confirmed 625 arrivals on eight small boats yesterday.