Six persons including five of a family were killed on Monday in violence over land in Fatehpur village of Rudrapur area here, police said.
The incident took place in the Lehda Tola locality in the morning, they said, adding that 14 people have been detained so far.
Two companies of the PAC have been deployed in the village, a senior official said.
Opposition Samajwadi Party and Congress attacked the Yogi Adityanath government over the incident and demanded a high-level probe into the matter.
Former district panchayat member Prem Yadav, 50, was attacked with sharp-edged weapons and killed by his rival Satyaprakash Dubey and his family when he went to his house, Special DG, Law and Order, Prashant Kumar said.
In retaliation soon after, Yadav's supporters from Abhaipur attacked Dubey's house and brutally killed Dubey and five of his family, including children.
Besides 54-year-old Dubey, his wife Kiran Dubey, 52, daughters Saloni, 18, and Nandani, 10, and son Gandhi, 15, were killed in the attack.
Dubey's 8-year-old son Anmol was injured in the attack and has been referred to BRD Medical College, Gorakhpur, where his condition was stated to be serious.
The violence erupted out of a land dispute the two families were locked in.
Two persons have been arrested in connection with the killings, Kumar said.
District Magistrate, Deoria, Akhand Pratap Singh said they got information about the incident around 8 am.
"The incident was reported after an altercation between two groups. A person of a group was found dead and six others were found in an unconscious state, of whom five were declared dead by doctors at the hospital," he said.
"The cause behind the incident is stated to be a longstanding land dispute between two families of the village. The brother of Satya Prakash Dubey, Sadhu Dubey had sold his land to Premchand Yadav and the matter was resolved seven years ago," he said, adding that a probe is on in the matter.
Meanwhile, Special Director General of Police (Law and Order) Prashant Kumar and Principal Secretary (Home) Sanjay Prasad visited the site of the incident.
SDG (Law and Order) Kumar said, "It is an unfortunate incident, in which five persons from one side and one person from the other side were killed. The reason, which has been told is that there was a dispute over a piece of land, which Satyprakash's brother Gyanprakash Dubey had given to the other side and there was a dispute regarding this."
"So far, 14 persons have been detained, and concrete and effective action will be taken in the entire case. As you are saying that an application was given in the police station, and there was laxity regarding it, then action will also be initiated against the officials indulging in laxity," Kumar said.
He also said that stringent action will be initiated against those who took the law in their own hands.
Two companies of the PAC have been deployed in the village, and senior officers are present on the spot, he said.
Principal Secretary (Home) Prasad said, "I and SDG (Law and Order) have come here on the directives of the chief minister. Prima facie, the incident took place due to personal enmity. In all, six people have died."
"More information regarding the incident is being gathered. Since, all the family members of one side have died and some people of the other side are also not here, so it will take some more time to gather all the information. Fourteen persons have been detained for questioning."
Chief Minister Yogi Adityanath directed the Commissioner of Police and the Inspector General to take strict action against the guilty.
He also condoled with the bereaved members of the two families and directed officials to ensure the best treatment for the injured.
Bharatiya Janata Party MLA from Deoria Sadar, Shalabh Mani Tripathi, said he was saddened by the killings and would get the matter investigated.
"Heartbroken by the brutal incident that took place in Fatehpur of Rudrapur assembly constituency of Deoria. This is a very unfortunate and sad incident. This type of incident is absolutely unacceptable," Tripathi said.
"It will be thoroughly investigated and instructions have been given for strict action. The guilty will not be spared and accountability of people of revenue and police administration will also be fixed," he added.
Superintendent of Police Sankalp Sharma said heavy police deployment has been made in the village considering the sensitive matter involving two castes.
Demanding a high-level probe into the matter, SP chief Akhilesh on X said, 'The Deoria incident happened due to the failure of the government and negligence or involvement of the administration somewhere. I wish the chief minister's expression of grief could bring back people's lives.'
"Only a high-level investigation can uncover why this murder took place and provide justice. This investigation should be done immediately," he added.
Senior Aam Aadmi Party leader from UP Shekhar Dixit, while expressing concern over the incident, said, "It is indeed an unfortunate incident. And it seems that the law and order situation in Uttar Pradesh is deteriorating with every passing day."
Spokesperson of Rashtriya Lok Dal's UP unit Ankur Saxena said that 'the law and order mechanism in the state has virtually collapsed'.
Congress also took to X to attack the Yogi Adityanath government.
In a post from the official account of the UP Congress Committee, the party said, 'Six people have been killed in a land dispute in Rudrapur, Deoria. The law and order of the state has become so bad that even in small disputes, six people are killed openly without fear. There is neither fear of law, nor its respect.'
'The Yogi government has transformed this state into such a crime zone that women sitting at home, children, youth and elderly people are not safe,' the Congress said.