The assault took place in Telegraph Lane in Claygate around 1.15pm. Police were called after paramedics attended the location to treat the woman, who had suffered serious head injuries. It is believed that she may have been attacked with a hammer.
The woman was taken to hospital where she remains in a serious condition.
A 22-year-old local man was arrested on suspicion of attempted murder shortly after the incident. He remains in custody and enquiries are ongoing.
Temporary Detective Inspector Sean West, who is investigating the incident, said: “This appears to have been a completely unprovoked attack which has left the woman with serious head injuries. Such violent and senseless attacks are thankfully extremely rare in our local communities and I would like to reassure the public that we are working around the clock and doing everything possible to ensure the person responsible is brought to justice.
“We have already spoken to a number of witnesses and house to house enquiries are being carried out. However, we are urging anyone else who witnessed the attack, or who may have seen a man acting suspiciously in the area at the time of the incident, or with any other information, to come forward as a matter of urgency.
“Your information, no matter how insignificant it might seem, could prove vital to our investigation.”
If you have any information which could help, please contact us quoting crime reference number PR/45200100489 by calling on 101