The 24-year-old was pushing the buggy at about 7.30am on Monday, when a man approached her and pulled at her clothing.

The woman managed to fight him off, and he ran away towards the pedestrian crossing in Lady's Lane.

A spokeswoman for Northamptonshire Police said officers were following a number of lines of inquiry, including studying CCTV footage.

She added: "This was a disturbing assault on a woman pushing her child along a town centre underpass early on Monday and we would like anyone who witnessed this incident, or who has information about it, to contact us.

"The woman was left in a distressed state so if anyone noticed this man acting suspiciously in Lady's Lane, or in the vicinity of the Chronicle & Echo or Greyfriars bus Station at this time, then please get in touch as a matter of urgency."

The attacker was described as a light-skinned mixed-race man, 5ft 7in, stocky and aged in his late 30s to early 40s.

He had a round face, small eyes, and wore a lot of layers. He wore a dark grey woollen beanie hat which covered his hair, a dark-coloured fleece body warmer, a dirty jumper, which had dark red arms and possibly a white front with a motif on it, and dark red trousers.

Police are also investigating a separate attack in an underpass in Rickyard Road, Boothville, Northampton, which happened between 9.50am and 10.15am on Monday.

A 15-year-old girl was walking through the underpass when she was grabbed by the shoulders by a black man in his early 20s, but the victim managed to run away.

Her attacker was a 5ft 6ins, skinny, with short dark hair shaven around the sides and back.

He wore a black T-shirt which had a glittery pattern on it, dark blue three-quarter length jeans, red and white trainers and he carried a yellow and purple sports bag over his shoulder.

Anyone with information on either attack can contact Northamptonshire Police on 08453 700700 or Crimestoppers anonymously on 0800 555111.