A sleepy sidestreet near the centre of Maidstone may seem an unlikely frontline in the conflict that has bubbled away, usually with relative calm, since Britain legalised abortion in 1967.

But on a recent weekday afternoon in Kent's county town, a group of a dozen anti-abortion protesters, led by a veteran of the movement in the US, began their latest "prayer vigil" directly across the road from a Marie Stopes clinic.

Over the course of two hours, members of the group intercepted young women approaching the clinic from either end of the street to hand them literature and engage in conversation, while the protesters themselves became the target of shouts of "disgusting" and "shame" from angry passersby.

The protesters hail from the Helpers of God's Precious Infants anti-abortion group and are led by Monsignor Philip Reilly, who has travelled from the US to meet British supporters.

Pro-choice groups say the Maidstone protest reflects an apparent ratcheting up of the activities of the more active elements in the anti-abortion movement, typically involving individuals with experience of the polarised world of America's "culture wars".

As well as Marie Stopes clinics around the UK, targets have included branches of the British Pregnancy Advisory Service (BPAS) and the sexual health charity Brook.

The surge in prayer vigil protests comes as abortion is edging back to the centre of mainstream political discourse.

A row erupted last week when the Guardian revealed an anti-abortion charity, Life, had effectively replaced the BPAS on a new sexual health advisory panel, while the MPs Nadine Dorries and Frank Field are spearheading a new drive in parliament to tighten the rules on terminations as part of the health and social care bill.

In Maidstone, police arrived to tell the group to remain on the other side of the street from the clinic. While two of the protesters engaged in an animated conversation with the officers, denying they were obstructing or harassing clients, a car with two women who had left the clinic accelerated at high speed and veered precariously close to the protest before driving on.

Reilly appeared unmoved by the suggestion he and his supporters were upsetting women already in the midst of a difficult personal experience.

"If you are upset because you are about to kill your child, because someone is outside praying, well thanks be to God that they are upset because maybe they will change their mind, keep their child and thank us later," he told the Guardian.

Backing him are three bishops from the British Catholic hierarchy who have endorsed the vigils carried out by the Helpers of God's Precious Infants, including Thomas McMahon, the bishop of Brentwood.

Darinka Aleksic, the campaign co-ordinator at Abortion Rights, the national pro-choice campaign, said: "We need to keep it in perspective because, in comparison with the US where there is a massive amount of harassment and threats to abortion providers, our situation is much better and we can be grateful."

But she admitted there had been a recent rise in the number of American anti-abortion groups setting up UK branches: "There have always been pickets in the UK, particularly in Northern Ireland, but there has not been so much of it here.

"We don't stage counter-protests because we don't want to give them more publicity and/or turn the street into a battleground. But anything that makes it more distressing for women who have to walk past is very worrying."

A previous lack of co-operation between different parts of the anti-abortion movement is changing, according to Andy Stephenson of Abort 67, which deploys shocking images to try to deter women from going through with terminations.

"A younger generation of anti-abortion leaders are emerging with more willingness to collaborate than has been seen before," said Stephenson, who was arrested twice outside the Wistons abortion clinic in Brighton last September on suspicion of a public order offence for holding a large banner depicting an aborted foetus. The Crown Prosecution Service took no action and the activists involved, whose case was supported by the Christian Legal Centre, say they now have "very significant plans" for later in the year.

Other groups include the UK chapter of Bound4Life, which has imported from the US the tactic of "silent sieges" with activists standing outside abortion clinics with their mouths covered in red tape on which the word "life" is written.

Meanwhile, prayer vigils are held outside clinics by the 40 Days for Life campaign, which is affiliated to a US anti-abortion network and whose activities have been endorsed by the much older Society for the Protection of Unborn Children. Since the campaign started in the UK last September, 40-day vigils have been held outside a number of clinics and more are planned for later this year.

The group has been accused of filming women and clinic employees and handing out leaflets containing warnings about a supposed increase in breast cancer among women who have abortions. The protesters say they film their actions to protect themselves from attack.

The campaign's director, Robert Colquhoun, said: "Many people perceive us to be fundamentalist, judgmental Christians but, through our prayer vigils, we have encountered many women who were going to go for abortions but who, having had the offer of help and alternatives from us, have decided not to go for abortion as a result of our peaceful, prayerful and legal presence there."

Not surprisingly, such activites are viewed rather differently inside the clinics.

Inside Maidstone's Marie Stopes centre, the manager, Julie Wilson, said: "The clinic has been here for 11 years and we have had protesters outside the building on a regular basis.

"Usually they are peaceful although, on occasions, they can be more intrusive and clients can be upset and decide not to go ahead with their appointment on the day. Generally though, we find these clients come back. It's actually the people who accompany the clients who are often more upset."