European stocks closed lower on Tuesday, as investors erred on the side of caution ahead of potentially market-moving events later this week, including the U.K.'s general election and a European Central Bank meeting.

U.S. stocks fell for a second session on Tuesday as market sentiment turned jittery ahead of a U.K. general election and uncertainties associated with former Federal Bureau of Investigation boss James Comey's testimony to the Senate later this week.

Asian equity markets lacked direction early Wednesday, as investors retreated to the sidelines ahead of bellwether global events this week. The planned public testimony by former FBI Director James Comey on Thursday continued to stoke concerns about U.S. political instability. Additionally, investors' showed their nerves ahead of a European Central Bank policy meeting and the U.K. general election, both scheduled for Thursday.