French police arrested more than 7,400 migrants in Calais trying to make their way across the Channel to Britain in the first six months of this year.
The figure is more than double the 3,129 arrested in the same period last year. In the first fortnight of July alone officers detained 1,200 migrants in Calais, the local police prefecture said on Friday.
Most of the migrants came from the Horn of Africa, including Somalia, Sudan and Eritrea and many appeared to have arrived in Europe via Lampedusa in Italy, officials said.
The police prefecture said the increase in arrests was down to a combination of a growing flux of migrants gathering in the port area and better detection methods.
One Calais officer told French journalists: "We have noticed that there are considerably more illegal migrants, and during our checks we find more of them."
Humanitarian groups have been demanding that France rethink its policy towards migrants. In recent months police have dismantled camps around the port and last week the local mayor issued a decree banning camps from certain areas because he claimed there were public order and health issues.
In May the French authorities bulldozed three makeshift migrant camps sheltering hundreds of people at Calais, citing health concerns following an outbreak of scabies and increasing violence.
The authorities said the area needed clearing because of "deplorable" hygiene conditions.
However, aid organisations said arrests, bulldozers and expulsions were not the answer.
Jean-Claude Lenoir, president of Salam, which has been offering meals to the migrants, said: "As if the situation isn't tragic enough, iIt's intolerable that once again these expulsions are being carried out without any alternative being proposed."
At the three most popular sites for migrants to attempt a Channel crossing, Calais, Dunkirk and the Channel Tunnel, a total of 10,500 migrants were arrested in the first six months of 2014, compared with 5,133 for the same period in 2013.
