Just hours later, the party's website and other online services came under a second digital bombardment, according to two people with knowledge of the matter and documents seen by Reuters.
The sources said it was unclear if it was the same hackers or a copycat attack but there was currently nothing to link either incident to a foreign state.
Asked about the second attack, a Labour spokesman said: "We have ongoing security processes in place to protect our platforms, so users may be experiencing some differences. We are dealing with this quickly and efficiently."
Britain's security agencies have warned that Russia and other countries could use cyberattacks or political messages on social media to attempt to disrupt the Dec. 12 election.
Moscow has repeatedly denied Western allegations of election interference.
Britain's National Cyber Security Centre, part of the GCHQ signals intelligence agency, said the first attack was a distributed denial-of-service (DDoS) attack - a technique used by hackers to take down websites by overwhelming them with traffic.
"DDoS attacks are a common form of attack used by a very wide range of attackers. Mitigation techniques are available and worked in this case," a NCSC spokesman said.
The nature of such attacks often made it difficult to attribute responsibility to any particular group, he said.
'VERY NERVOUS'
Labour leader Jeremy Corbyn said the first attack was very serious but was successfully repelled by the party's defence systems when the digital assault began on Monday.
"But if this is a sign of things to come in this election, I feel very nervous about it all," he said. "Because a cyber attack against a political party in an election is suspicious and something one is very worried about."
A Labour spokesman said that while the attack on Monday had slowed down some campaign activity, they were restored on Tuesday.
Britain goes to the polls on Dec. 12 in an election called by Prime Minister Boris Johnson to try to break the Brexit deadlock in parliament more than three years since the country voted to leave the European Union.
A report by parliament's Intelligence and Security Committee has investigated Russian activity in British politics and reportedly includes charges of spying and interference in polls, including the 2016 Brexit referendum and the 2017 national election.
The government, however, has declined to publish it before the upcoming election.