The watchdog investigated two separate allegations, featuring in the Daily Telegraph and Channel 4's Dispatches.
It said the alleged offences were "ultimately a criminal matter" and fell outside its remit.
The Lib Dems said they would "continue to cooperate with any investigation".
Liberal Democrat peer Lord Strasburger of Langridge resigned from the party's group in the Lords ahead of the Dispatches broadcast, and former fundraiser Ibrahim Taguri stepped down as a party candidate after the Telegraph claimed he accepted a "potentially illegal donation".
Both men deny any wrongdoing.
The Electoral Commission said the two alleged offences centred on possible breaches of the Political Parties, Elections and Referendums Act 2000.
For this reason, it said the matter fell outside its jurisdiction and referred it to the police.
The Met confirmed it had been "liaising with the Electoral Commission regarding possible irregularities in relation to political party donations", saying two matters were being assessed by officers from the "Special Enquiry Team".
A Liberal Democrat spokesman said: "When the party was notified of the allegations, we immediately referred them to the Electoral Commission and have fully complied with their inquiries.
"We will continue to cooperate with any investigation."
The Electoral Commission says it has referred two allegations that the Liberal Democrats received donations in breach of party funding rules to the Metropolitan Police.