Fidelity trading platforms experienced outages just as U.S. stock markets opened for trading Monday morning, leaving many users of its online, mobile and Active Trader apps unable to execute buy or sell orders.

The outage occurred even as major share prices index soared on news of a breakthrough in U.S.-China trade talks over the weekend, leaving many investors frustrated that they couldn't participate in the rally.

The Standard & Poor's 500 index was up more than 2.5% by 10:30 a.m. ET.

"The volume today is the highest at this point in the day" out of any trading so far in May, said Steve Sosnick, chief market strategist at Interactive Brokers.

"We are working urgently on resolving the issues," Fidelity said in a post on Reddit, the site of many user complaints. "We apologize for  the  inconvenience and appreciate you being  a  customer."

Fidelity declined any further comment on the outages.

Thousands of users reported being unable to access their accounts as of mid-morning on Monday, according to outages tracking website Down Detector.