Customers attempting to reserve the next iPhone were thwarted by technical problems on the websites of Apple Inc and AT&T Inc on Tuesday, the first day of pre-orders for the phone.

AT&T, the iPhone's exclusive U.S. operator, declined to comment, and Apple did not respond to a questions about the website issues that provoked complaints by customers in Los Angeles, San Francisco and New York.

Frustrated by website problems, customers lined up outside some stores to ask AT&T employees to manually process their orders for the iPhone 4.

One attempt by a Reuters reporter to pre-order the phone on both Apple and AT&T's websites generated error messages. The following came from AT&T's website:

There was a problem with your request, it read. We're sorry, but we are experiencing a system error that prevents us from completing your request.

Apple's website said the request could not be processed, and it suggested the customer try again later. Similar messages continued to appear Tuesday afternoon.

The technical problems marked another headache for AT&T, which last week said some iPad users had personal information exposed via a network security flaw. The Federal Bureau of Investigation has opened a probe into the breach.

Brian Marshall, an analyst for Gleacher & Co, formerly known as Broadpoint Amtech, said AT&T should have been ready for the rush of iPhone pre-orders.

Marshall, who himself experienced problems on the site, estimated that tens of thousands if not hundreds of thousands are going online at the same time to place orders.

As a consumer I'm annoyed, he said. I think they should have been more prepared for the onslaught of demand.

At a Los Angeles AT&T store, customer Bobby Hicks said he decided to try to order in person after failing online. The filmmaker said he had been waiting 35 minutes.

Customers waiting at AT&T stores in San Francisco and New York voiced similar complaints.

In San Francisco, about 20 people waited in a mid-morning line that ran outside an AT&T wireless store.

It appeared that orders were being processed at the store, but an AT&T clerk said the systems had been overwhelmed and were running more slowly than usual.

Kevin Wong, a systems engineer waiting at the San Francisco store, said he had been in line for about 20 minutes after he was unable to buy the device online.

It bothers me, but what can you do? You've got to wait in line. It's better than the DMV, he said referring to the state's Department of Motor Vehicles, which is notorious for long wait times.

Earlier this month, Apple unveiled the iPhone 4, which will go on sale in Apple and AT&T stores on June 24. Pre-ordering began on Tuesday.

The redesigned iPhone starts at $199, and is slimmer than the current handset and boasts a higher quality screen and better battery life. It will allow video chat via Wi-Fi for the first time and has a gyroscope sensor for improved gaming.

Gleacher's Marshall said the online overload may be partly due to consumers becoming smarter about ordering online rather than camping out for hours or days at stores to buy iPhone.

People are a little bit wiser about efficient use of time, he said.

Apple shares rose $5.41 at $259.69 on the Nasdaq. AT&T shares rose 37 cents at $25.54 on New York Stock Exchange.

(Additional reporting by Caroline Madrid in Los Angeles and Paul Thomasch in New York; Editing by Steve Orlofsky, Leslie Gevirtz)