British police arrested nine people on Saturday in the Channel port city of Dover after clashes with anti-immigration activists.

The arrests followed dozens of demonstrators blocking a four-lane road near Dover, and as pro-migrant protesters held their own rally in the southeast English city.

The incidents come as the number of migrants crossing the Straits of Dover -- which is 21 miles (33.8 kilometres) at the closest point -- in small inflatable boats has spiralled over the summer. 

"Officers have made nine arrests during today’s protests in #Dover, including for racially aggravated public order, violent disorder and assaulting an emergency worker," Kent police said on Twitter.

"Enquiries are ongoing to identify any further offences."

Footage posted on social media showed officers scuffling with several anti-migrant activists as they made arrests near Dover's busy harbour. 

Many protesters wore Union Jack flag face masks and carried banners decrying illegal immigration across the Channel to Britain.

Some shouted "England till I die" and sang the popular patriotic song "Rule, Britannia!".

The group of several dozen demonstrators, thought to have come from far beyond Dover, gathered after record numbers of migrants have tried to cross the Channel this week in the small boats.

Tuesday saw more than 400 migrants make the dangerous journey across the world's busiest shipping lane -- thought to be a record for a single day.

Meanwhile, nearly 1,500 migrants and asylum-seekers arrived in Britain by small boats in August, according to an analysis by the domestic Press Association news agency.

Around 100 people attended the earlier protest in support of asylum-seekers and refugees.

"We are standing up and welcoming people who are in desperate circumstances fleeing from awful situations," said Peter Keenan, from the Kent Refugee Help group.