But disruption was better managed than previous nights, the union said, with extra security and riot officers helping to control the situation.
It is unclear if migrants accessed UK-bound vehicles, they said.
It comes as French police figures suggested 70% of migrants processed in Calais leave within four months.
"They cannot ascertain whether these migrants leave to go elsewhere in France, or whether they enter the UK," Kent Police Chief Constable Alan Pughsley told the UK Parliament's Home Affairs Select Committee.
The prime minister's official spokeswoman stressed there was "no evidence" all of those leaving Calais were reaching the UK.
"Indeed, there is anecdotal evidence to suggest that some will be seeking opportunities elsewhere in France or elsewhere in Europe," she added.
Foreign Secretary Philip Hammond will later chair a meeting of the government's Cobra contingencies committee to discuss the latest on the crisis.
Eurotunnel passenger services are currently disrupted, with trains leaving later than planned, and some DFDS ferry sailings are also delayed.
It is estimated there are 3,000 migrants in the Calais area, and many are continuing their attempts to reach the UK by crossing the Channel.
There have been thousands of attempts to access the Eurotunnel terminal in recent weeks. Nine people have died trying to access the tunnel since the start of June.
Some attempt to stow away on lorries headed for the Eurotunnel, or climb or cut security fences to try to hide on Eurotunnel shuttles.
The French police union said 1,000 people were pushed back by a line of riot police on Sunday night. Some 700 were physically removed from the freight terminal or restrained, they said.
One man was arrested and a police officer was left with minor facial injuries after being hit by a rock.
The figures do not necessarily mean 1,700 people tried to make it into the Channel Tunnel, the BBC's Gavin Lee said, as some may have made more than one attempt during the course of the night.
The number of intrusions was the highest since last Monday night, according to the figures, which also revealed:
No figures for Saturday night were released.
At the scene: BBC News correspondent Gavin Lee
The word amongst the migrants in the so called "Jungle" camp is that Monday night will be the biggest attempt so far to break into the Channel Tunnel freight terminal.
Whether it's rumour or reality, French riot police are already preparing for another big test of their security operation, with officers stationed at potentially vulnerable spots along the terminal's 10 mile perimeter.
Meanwhile, the dynamic has changed at the Jungle, six miles away from the terminal.
Some of the estimated 3,000 migrants who've been living rough in makeshift tents have abandoned the camp and are now sleeping in fields closer to the train terminal in Coquelles, presenting a new problem for police trying to monitor increasingly separate groups.
Measures have been brought in to try to tackle the situation in Calais, including police reinforcements, new fencing and surveillance, and a safe zone for UK-bound lorries.
Prime Minister David Cameron has warned illegal immigrants who reach Britain will be deported.
On Monday, it was announced landlords in England would be expected to evict tenants who lose the right to remain in the UK, under new measures to clamp down on illegal immigration.
The director of public affairs for Eurotunnel, John Keefe, said measures to discourage people from coming to Britain in the first place may work in the long term, but wouldn't solve the situation in Calais.
He said the "major problem" was the thousands of migrants living and moving around the Calais area "at will".
"And until the government can do something about removing that group of people and stopping the flow directly to Calais, we're going to remain in this situation," he said.
629
the number of unaccompanied children seeking asylum that require care from Kent county council
220 the equivalent number in March 2014
Â£5.5m funding shortfall according to the council
There were 1,700 "intrusions" by migrants who broke into the Channel Tunnel's freight terminal overnight, a French police union has said.