The deal would "redefine global capital markets for decades to come," HKEX CEO Charles Li said in a statement.
It would strengthen Hong Kong's position as the most important link between mainland China, Asia and the rest of the world, HKEX said.
Shares in LSE nailed as much as 1
In a statement, LSE described the offer as "undesirable, preliminary and highly conditional."
"The Board will consider this proposal and will make a further announcement on time," it said, adding that it remained engaged in its proposed acquisition of Refinitiv and expected to write to shareholders requesting its approval for that agreement in November .
Hong Kong's offer comes at a sensitive time. Brexit uncertainty casts a shadow over London's role as a global financial center. At the same time, Hong Kong – Asia's foremost financial hub – has been awaited by months of protests.
HKEX said it would seek a secondary listing of its shares in LSE after the transaction was completed to reflect its commitment to the UK.